Data Center BGP: Use of BGP for Routing in Large-Scale Data Centers
Routing large server farms with BGP instead of traditional switching
Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of BGP for Routing in Large-Scale Data Centers", RFC 7938, DOI 10.17487/RFC7938, August 2016, <https://www.rfc-editor.org/info/rfc7938>.
@misc{rfc7938,
series = {Request for Comments},
number = 7938,
howpublished = {RFC 7938},
publisher = {RFC Editor},
doi = {10.17487/RFC7938},
url = {https://www.rfc-editor.org/info/rfc7938},
author = {P. Lapukhov and A. Premji and J. Mitchell},
title = {{Use of BGP for Routing in Large-Scale Data Centers}},
pagetotal = 35,
year = 2016,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) P. Lapukhov
Request for Comments: 7938 Facebook
Category: Informational A. Premji
ISSN: 2070-1721 Arista Networks
J. Mitchell, Ed.
August 2016
Use of BGP for Routing in Large-Scale Data Centers
Abstract
Some network operators build and operate data centers that support
over one hundred thousand servers. In this document, such data
centers are referred to as "large-scale" to differentiate them from
smaller infrastructures. Environments of this scale have a unique
set of network requirements with an emphasis on operational
simplicity and network stability. This document summarizes
operational experience in designing and operating large-scale data
centers using BGP as the only routing protocol. The intent is to
report on a proven and stable routing design that could be leveraged
by others in the industry.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7938.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Network Design Requirements . . . . . . . . . . . . . . . . . 4
2.1. Bandwidth and Traffic Patterns . . . . . . . . . . . . . 4
2.2. CAPEX Minimization . . . . . . . . . . . . . . . . . . . 4
2.3. OPEX Minimization . . . . . . . . . . . . . . . . . . . . 5
2.4. Traffic Engineering . . . . . . . . . . . . . . . . . . . 5
2.5. Summarized Requirements . . . . . . . . . . . . . . . . . 6
3. Data Center Topologies Overview . . . . . . . . . . . . . . . 6
3.1. Traditional DC Topology . . . . . . . . . . . . . . . . . 6
3.2. Clos Network Topology . . . . . . . . . . . . . . . . . . 7
3.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . 7
3.2.2. Clos Topology Properties . . . . . . . . . . . . . . 8
3.2.3. Scaling the Clos Topology . . . . . . . . . . . . . . 9
3.2.4. Managing the Size of Clos Topology Tiers . . . . . . 10
4. Data Center Routing Overview . . . . . . . . . . . . . . . . 11
4.1. L2-Only Designs . . . . . . . . . . . . . . . . . . . . . 11
4.2. Hybrid L2/L3 Designs . . . . . . . . . . . . . . . . . . 12
4.3. L3-Only Designs . . . . . . . . . . . . . . . . . . . . . 12
5. Routing Protocol Design . . . . . . . . . . . . . . . . . . . 13
5.1. Choosing EBGP as the Routing Protocol . . . . . . . . . . 13
5.2. EBGP Configuration for Clos Topology . . . . . . . . . . 15
5.2.1. EBGP Configuration Guidelines and Example ASN Scheme 15
5.2.2. Private Use ASNs . . . . . . . . . . . . . . . . . . 16
5.2.3. Prefix Advertisement . . . . . . . . . . . . . . . . 17
5.2.4. External Connectivity . . . . . . . . . . . . . . . . 18
5.2.5. Route Summarization at the Edge . . . . . . . . . . . 19
6. ECMP Considerations . . . . . . . . . . . . . . . . . . . . . 20
6.1. Basic ECMP . . . . . . . . . . . . . . . . . . . . . . . 20
6.2. BGP ECMP over Multiple ASNs . . . . . . . . . . . . . . . 21
6.3. Weighted ECMP . . . . . . . . . . . . . . . . . . . . . . 21
6.4. Consistent Hashing . . . . . . . . . . . . . . . . . . . 22
7. Routing Convergence Properties . . . . . . . . . . . . . . . 22
7.1. Fault Detection Timing . . . . . . . . . . . . . . . . . 22
7.2. Event Propagation Timing . . . . . . . . . . . . . . . . 23
7.3. Impact of Clos Topology Fan-Outs . . . . . . . . . . . . 24
7.4. Failure Impact Scope . . . . . . . . . . . . . . . . . . 24
7.5. Routing Micro-Loops . . . . . . . . . . . . . . . . . . . 26
8. Additional Options for Design . . . . . . . . . . . . . . . . 26
8.1. Third-Party Route Injection . . . . . . . . . . . . . . . 26
8.2. Route Summarization within Clos Topology . . . . . . . . 27
8.2.1. Collapsing Tier 1 Devices Layer . . . . . . . . . . . 27
8.2.2. Simple Virtual Aggregation . . . . . . . . . . . . . 29
8.3. ICMP Unreachable Message Masquerading . . . . . . . . . . 29
9. Security Considerations . . . . . . . . . . . . . . . . . . . 30
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Normative References . . . . . . . . . . . . . . . . . . 30
10.2. Informative References . . . . . . . . . . . . . . . . . 31
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction
This document describes a practical routing design that can be used
in a large-scale data center (DC) design. Such data centers, also
known as "hyper-scale" or "warehouse-scale" data centers, have a
unique attribute of supporting over a hundred thousand servers. In
order to accommodate networks of this scale, operators are revisiting
networking designs and platforms to address this need.
The design presented in this document is based on operational
experience with data centers built to support large-scale distributed
software infrastructure, such as a web search engine. The primary
requirements in such an environment are operational simplicity and
network stability so that a small group of people can effectively
support a significantly sized network.
Experimentation and extensive testing have shown that External BGP
(EBGP) [RFC4271] is well suited as a stand-alone routing protocol for
these types of data center applications. This is in contrast with
more traditional DC designs, which may use simple tree topologies and
rely on extending Layer 2 (L2) domains across multiple network
devices. This document elaborates on the requirements that led to
this design choice and presents details of the EBGP routing design as
well as exploring ideas for further enhancements.
This document first presents an overview of network design
requirements and considerations for large-scale data centers. Then,
traditional hierarchical data center network topologies are
contrasted with Clos networks [CLOS1953] that are horizontally scaled
out. This is followed by arguments for selecting EBGP with a Clos
topology as the most appropriate routing protocol to meet the
requirements and the proposed design is described in detail.
Finally, this document reviews some additional considerations and
design options. A thorough understanding of BGP is assumed by a
reader planning on deploying the design described within the
document.
2. Network Design Requirements
This section describes and summarizes network design requirements for
large-scale data centers.
2.1. Bandwidth and Traffic Patterns
The primary requirement when building an interconnection network for
a large number of servers is to accommodate application bandwidth and
latency requirements. Until recently it was quite common to see the
majority of traffic entering and leaving the data center, commonly
referred to as "north-south" traffic. Traditional "tree" topologies
were sufficient to accommodate such flows, even with high
oversubscription ratios between the layers of the network. If more
bandwidth was required, it was added by "scaling up" the network
elements, e.g., by upgrading the device's linecards or fabrics or
replacing the device with one with higher port density.
Today many large-scale data centers host applications generating
significant amounts of server-to-server traffic, which does not
egress the DC, commonly referred to as "east-west" traffic. Examples
of such applications could be computer clusters such as Hadoop
[HADOOP], massive data replication between clusters needed by certain
applications, or virtual machine migrations. Scaling traditional
tree topologies to match these bandwidth demands becomes either too
expensive or impossible due to physical limitations, e.g., port
density in a switch.
2.2. CAPEX Minimization
The Capital Expenditures (CAPEX) associated with the network
infrastructure alone constitutes about 10-15% of total data center
expenditure (see [GREENBERG2009]). However, the absolute cost is
significant, and hence there is a need to constantly drive down the
cost of individual network elements. This can be accomplished in two
ways:
o Unifying all network elements, preferably using the same hardware
type or even the same device. This allows for volume pricing on
bulk purchases and reduced maintenance and inventory costs.
o Driving costs down using competitive pressures, by introducing
multiple network equipment vendors.
In order to allow for good vendor diversity, it is important to
minimize the software feature requirements for the network elements.
This strategy provides maximum flexibility of vendor equipment
choices while enforcing interoperability using open standards.
2.3. OPEX Minimization
Operating large-scale infrastructure can be expensive as a larger
amount of elements will statistically fail more often. Having a
simpler design and operating using a limited software feature set
minimizes software issue-related failures.
An important aspect of Operational Expenditure (OPEX) minimization is
reducing the size of failure domains in the network. Ethernet
networks are known to be susceptible to broadcast or unicast traffic
storms that can have a dramatic impact on network performance and
availability. The use of a fully routed design significantly reduces
the size of the data-plane failure domains, i.e., limits them to the
lowest level in the network hierarchy. However, such designs
introduce the problem of distributed control-plane failures. This
observation calls for simpler and less control-plane protocols to
reduce protocol interaction issues, reducing the chance of a network
meltdown. Minimizing software feature requirements as described in
the CAPEX section above also reduces testing and training
requirements.
2.4. Traffic Engineering
In any data center, application load balancing is a critical function
performed by network devices. Traditionally, load balancers are
deployed as dedicated devices in the traffic forwarding path. The
problem arises in scaling load balancers under growing traffic
demand. A preferable solution would be able to scale the load-
balancing layer horizontally, by adding more of the uniform nodes and
distributing incoming traffic across these nodes. In situations like
this, an ideal choice would be to use network infrastructure itself
to distribute traffic across a group of load balancers. The
combination of anycast prefix advertisement [RFC4786] and Equal Cost
Multipath (ECMP) functionality can be used to accomplish this goal.
To allow for more granular load distribution, it is beneficial for
the network to support the ability to perform controlled per-hop
traffic engineering. For example, it is beneficial to directly
control the ECMP next-hop set for anycast prefixes at every level of
the network hierarchy.
2.5. Summarized Requirements
This section summarizes the list of requirements outlined in the
previous sections:
o REQ1: Select a topology that can be scaled "horizontally" by
adding more links and network devices of the same type without
requiring upgrades to the network elements themselves.
o REQ2: Define a narrow set of software features/protocols supported
by a multitude of networking equipment vendors.
o REQ3: Choose a routing protocol that has a simple implementation
in terms of programming code complexity and ease of operational
support.
o REQ4: Minimize the failure domain of equipment or protocol issues
as much as possible.
o REQ5: Allow for some traffic engineering, preferably via explicit
control of the routing prefix next hop using built-in protocol
mechanics.
3. Data Center Topologies Overview
This section provides an overview of two general types of data center
designs -- hierarchical (also known as "tree-based") and Clos-based
network designs.
3.1. Traditional DC Topology
In the networking industry, a common design choice for data centers
typically looks like an (upside down) tree with redundant uplinks and
three layers of hierarchy namely; core, aggregation/distribution, and
access layers (see Figure 1). To accommodate bandwidth demands, each
higher layer, from the server towards DC egress or WAN, has higher
port density and bandwidth capacity where the core functions as the
"trunk" of the tree-based design. To keep terminology uniform and
for comparison with other designs, in this document these layers will
be referred to as Tier 1, Tier 2 and Tier 3 "tiers", instead of core,
aggregation, or access layers.
+------+ +------+
| | | |
| |--| | Tier 1
| | | |
+------+ +------+
| | | |
+---------+ | | +----------+
| +-------+--+------+--+-------+ |
| | | | | | | |
+----+ +----+ +----+ +----+
| | | | | | | |
| |-----| | | |-----| | Tier 2
| | | | | | | |
+----+ +----+ +----+ +----+
| | | |
| | | |
| +-----+ | | +-----+ |
+-| |-+ +-| |-+ Tier 3
+-----+ +-----+
| | | | | |
<- Servers -> <- Servers ->
Figure 1: Typical DC Network Topology
Unfortunately, as noted previously, it is not possible to scale a
tree-based design to a large enough degree for handling large-scale
designs due to the inability to be able to acquire Tier 1 devices
with a large enough port density to sufficiently scale Tier 2. Also,
continuous upgrades or replacement of the upper-tier devices are
required as deployment size or bandwidth requirements increase, which
is operationally complex. For this reason, REQ1 is in place,
eliminating this type of design from consideration.
3.2. Clos Network Topology
This section describes a common design for horizontally scalable
topology in large-scale data centers in order to meet REQ1.
3.2.1. Overview
A common choice for a horizontally scalable topology is a folded Clos
topology, sometimes called "fat-tree" (for example, [INTERCON] and
[ALFARES2008]). This topology features an odd number of stages
(sometimes known as "dimensions") and is commonly made of uniform
elements, e.g., network switches with the same port count.
Therefore, the choice of folded Clos topology satisfies REQ1 and
facilitates REQ2. See Figure 2 below for an example of a folded
3-stage Clos topology (3 stages counting Tier 2 stage twice, when
tracing a packet flow):
+-------+
| |----------------------------+
| |------------------+ |
| |--------+ | |
+-------+ | | |
+-------+ | | |
| |--------+---------+-------+ |
| |--------+-------+ | | |
| |------+ | | | | |
+-------+ | | | | | |
+-------+ | | | | | |
| |------+-+-------+-+-----+ | |
| |------+-+-----+ | | | | |
| |----+ | | | | | | | |
+-------+ | | | | | | ---------> M links
Tier 1 | | | | | | | | |
+-------+ +-------+ +-------+
| | | | | |
| | | | | | Tier 2
| | | | | |
+-------+ +-------+ +-------+
| | | | | | | | |
| | | | | | ---------> N Links
| | | | | | | | |
O O O O O O O O O Servers
Figure 2: 3-Stage Folded Clos Topology
This topology is often also referred to as a "Leaf and Spine"
network, where "Spine" is the name given to the middle stage of the
Clos topology (Tier 1) and "Leaf" is the name of input/output stage
(Tier 2). For uniformity, this document will refer to these layers
using the "Tier n" notation.
3.2.2. Clos Topology Properties
The following are some key properties of the Clos topology:
o The topology is fully non-blocking, or more accurately non-
interfering, if M >= N and oversubscribed by a factor of N/M
otherwise. Here M and N is the uplink and downlink port count
respectively, for a Tier 2 switch as shown in Figure 2.
o Utilizing this topology requires control and data-plane support
for ECMP with a fan-out of M or more.
o Tier 1 switches have exactly one path to every server in this
topology. This is an important property that makes route
summarization dangerous in this topology (see Section 8.2 below).
o Traffic flowing from server to server is load balanced over all
available paths using ECMP.
3.2.3. Scaling the Clos Topology
A Clos topology can be scaled either by increasing network element
port density or by adding more stages, e.g., moving to a 5-stage
Clos, as illustrated in Figure 3 below:
Tier 1
+-----+
Cluster | |
+----------------------------+ +--| |--+
| | | +-----+ |
| Tier 2 | | | Tier 2
| +-----+ | | +-----+ | +-----+
| +-------------| DEV |------+--| |--+--| |-------------+
| | +-----| C |------+ | | +--| |-----+ |
| | | +-----+ | +-----+ +-----+ | |
| | | | | |
| | | +-----+ | +-----+ +-----+ | |
| | +-----------| DEV |------+ | | +--| |-----------+ |
| | | | +---| D |------+--| |--+--| |---+ | | |
| | | | | +-----+ | | +-----+ | +-----+ | | | |
| | | | | | | | | | | |
| +-----+ +-----+ | | +-----+ | +-----+ +-----+
| | DEV | | DEV | | +--| |--+ | | | |
| | A | | B | Tier 3 | | | Tier 3 | | | |
| +-----+ +-----+ | +-----+ +-----+ +-----+
| | | | | | | | | |
| O O O O | O O O O
| Servers | Servers
+----------------------------+
Figure 3: 5-Stage Clos Topology
The small example of topology in Figure 3 is built from devices with
a port count of 4. In this document, one set of directly connected
Tier 2 and Tier 3 devices along with their attached servers will be
referred to as a "cluster". For example, DEV A, B, C, D, and the
servers that connect to DEV A and B, on Figure 3 form a cluster. The
concept of a cluster may also be a useful concept as a single
deployment or maintenance unit that can be operated on at a different
frequency than the entire topology.
In practice, Tier 3 of the network, which is typically Top-of-Rack
switches (ToRs), is where oversubscription is introduced to allow for
packaging of more servers in the data center while meeting the
bandwidth requirements for different types of applications. The main
reason to limit oversubscription at a single layer of the network is
to simplify application development that would otherwise need to
account for multiple bandwidth pools: within rack (Tier 3), between
racks (Tier 2), and between clusters (Tier 1). Since
oversubscription does not have a direct relationship to the routing
design, it is not discussed further in this document.
3.2.4. Managing the Size of Clos Topology Tiers
If a data center network size is small, it is possible to reduce the
number of switches in Tier 1 or Tier 2 of a Clos topology by a factor
of two. To understand how this could be done, take Tier 1 as an
example. Every Tier 2 device connects to a single group of Tier 1
devices. If half of the ports on each of the Tier 1 devices are not
being used, then it is possible to reduce the number of Tier 1
devices by half and simply map two uplinks from a Tier 2 device to
the same Tier 1 device that were previously mapped to different Tier
1 devices. This technique maintains the same bandwidth while
reducing the number of elements in Tier 1, thus saving on CAPEX. The
tradeoff, in this example, is the reduction of maximum DC size in
terms of overall server count by half.
In this example, Tier 2 devices will be using two parallel links to
connect to each Tier 1 device. If one of these links fails, the
other will pick up all traffic of the failed link, possibly resulting
in heavy congestion and quality of service degradation if the path
determination procedure does not take bandwidth amount into account,
since the number of upstream Tier 1 devices is likely wider than two.
To avoid this situation, parallel links can be grouped in link
aggregation groups (LAGs), e.g., [IEEE8023AD], with widely available
implementation settings that take the whole "bundle" down upon a
single link failure. Equivalent techniques that enforce "fate
sharing" on the parallel links can be used in place of LAGs to
achieve the same effect. As a result of such fate-sharing, traffic
from two or more failed links will be rebalanced over the multitude
of remaining paths that equals the number of Tier 1 devices. This
example is using two links for simplicity, having more links in a
bundle will have less impact on capacity upon a member-link failure.
4. Data Center Routing Overview
This section provides an overview of three general types of data
center protocol designs -- Layer 2 only, Hybrid Layer L2/L3, and
Layer 3 only.
4.1. L2-Only Designs
Originally, most data center designs used Spanning Tree Protocol
(STP) originally defined in [IEEE8021D-1990] for loop-free topology
creation, typically utilizing variants of the traditional DC topology
described in Section 3.1. At the time, many DC switches either did
not support Layer 3 routing protocols or supported them with
additional licensing fees, which played a part in the design choice.
Although many enhancements have been made through the introduction of
Rapid Spanning Tree Protocol (RSTP) in the latest revision of
[IEEE8021D-2004] and Multiple Spanning Tree Protocol (MST) specified
in [IEEE8021Q] that increase convergence, stability, and load-
balancing in larger topologies, many of the fundamentals of the
protocol limit its applicability in large-scale DCs. STP and its
newer variants use an active/standby approach to path selection, and
are therefore hard to deploy in horizontally scaled topologies as
described in Section 3.2. Further, operators have had many
experiences with large failures due to issues caused by improper
cabling, misconfiguration, or flawed software on a single device.
These failures regularly affected the entire spanning-tree domain and
were very hard to troubleshoot due to the nature of the protocol.
For these reasons, and since almost all DC traffic is now IP,
therefore requiring a Layer 3 routing protocol at the network edge
for external connectivity, designs utilizing STP usually fail all of
the requirements of large-scale DC operators. Various enhancements
to link-aggregation protocols such as [IEEE8023AD], generally known
as Multi-Chassis Link-Aggregation (M-LAG) made it possible to use
Layer 2 designs with active-active network paths while relying on STP
as the backup for loop prevention. The major downsides of this
approach are the lack of ability to scale linearly past two in most
implementations, lack of standards-based implementations, and the
added failure domain risk of syncing state between the devices.
It should be noted that building large, horizontally scalable,
L2-only networks without STP is possible recently through the
introduction of the Transparent Interconnection of Lots of Links
(TRILL) protocol in [RFC6325]. TRILL resolves many of the issues STP
has for large-scale DC design however, due to the limited number of
implementations, and often the requirement for specific equipment
that supports it, this has limited its applicability and increased
the cost of such designs.
Finally, neither the base TRILL specification nor the M-LAG approach
totally eliminate the problem of the shared broadcast domain that is
so detrimental to the operations of any Layer 2, Ethernet-based
solution. Later TRILL extensions have been proposed to solve the
this problem statement, primarily based on the approaches outlined in
[RFC7067], but this even further limits the number of available
interoperable implementations that can be used to build a fabric.
Therefore, TRILL-based designs have issues meeting REQ2, REQ3, and
REQ4.
4.2. Hybrid L2/L3 Designs
Operators have sought to limit the impact of data-plane faults and
build large-scale topologies through implementing routing protocols
in either the Tier 1 or Tier 2 parts of the network and dividing the
Layer 2 domain into numerous, smaller domains. This design has
allowed data centers to scale up, but at the cost of complexity in
managing multiple network protocols. For the following reasons,
operators have retained Layer 2 in either the access (Tier 3) or both
access and aggregation (Tier 3 and Tier 2) parts of the network:
o Supporting legacy applications that may require direct Layer 2
adjacency or use non-IP protocols.
o Seamless mobility for virtual machines that require the
preservation of IP addresses when a virtual machine moves to a
different Tier 3 switch.
o Simplified IP addressing = less IP subnets are required for the
data center.
o Application load balancing may require direct Layer 2 reachability
to perform certain functions such as Layer 2 Direct Server Return
(DSR). See [L3DSR].
o Continued CAPEX differences between L2- and L3-capable switches.
4.3. L3-Only Designs
Network designs that leverage IP routing down to Tier 3 of the
network have gained popularity as well. The main benefit of these
designs is improved network stability and scalability, as a result of
confining L2 broadcast domains. Commonly, an Interior Gateway
Protocol (IGP) such as Open Shortest Path First (OSPF) [RFC2328] is
used as the primary routing protocol in such a design. As data
centers grow in scale, and server count exceeds tens of thousands,
such fully routed designs have become more attractive.
Choosing a L3-only design greatly simplifies the network,
facilitating the meeting of REQ1 and REQ2, and has widespread
adoption in networks where large Layer 2 adjacency and larger size
Layer 3 subnets are not as critical compared to network scalability
and stability. Application providers and network operators continue
to develop new solutions to meet some of the requirements that
previously had driven large Layer 2 domains by using various overlay
or tunneling techniques.
5. Routing Protocol Design
In this section, the motivations for using External BGP (EBGP) as the
single routing protocol for data center networks having a Layer 3
protocol design and Clos topology are reviewed. Then, a practical
approach for designing an EBGP-based network is provided.
5.1. Choosing EBGP as the Routing Protocol
REQ2 would give preference to the selection of a single routing
protocol to reduce complexity and interdependencies. While it is
common to rely on an IGP in this situation, sometimes with either the
addition of EBGP at the device bordering the WAN or Internal BGP
(IBGP) throughout, this document proposes the use of an EBGP-only
design.
Although EBGP is the protocol used for almost all Inter-Domain
Routing in the Internet and has wide support from both vendor and
service provider communities, it is not generally deployed as the
primary routing protocol within the data center for a number of
reasons (some of which are interrelated):
o BGP is perceived as a "WAN-only, protocol-only" and not often
considered for enterprise or data center applications.
o BGP is believed to have a "much slower" routing convergence
compared to IGPs.
o Large-scale BGP deployments typically utilize an IGP for BGP next-
hop resolution as all nodes in the IBGP topology are not directly
connected.
o BGP is perceived to require significant configuration overhead and
does not support neighbor auto-discovery.
This document discusses some of these perceptions, especially as
applicable to the proposed design, and highlights some of the
advantages of using the protocol such as:
o BGP has less complexity in parts of its protocol design --
internal data structures and state machine are simpler as compared
to most link-state IGPs such as OSPF. For example, instead of
implementing adjacency formation, adjacency maintenance and/or
flow-control, BGP simply relies on TCP as the underlying
transport. This fulfills REQ2 and REQ3.
o BGP information flooding overhead is less when compared to link-
state IGPs. Since every BGP router calculates and propagates only
the best-path selected, a network failure is masked as soon as the
BGP speaker finds an alternate path, which exists when highly
symmetric topologies, such as Clos, are coupled with an EBGP-only
design. In contrast, the event propagation scope of a link-state
IGP is an entire area, regardless of the failure type. In this
way, BGP better meets REQ3 and REQ4. It is also worth mentioning
that all widely deployed link-state IGPs feature periodic
refreshes of routing information while BGP does not expire routing
state, although this rarely impacts modern router control planes.
o BGP supports third-party (recursively resolved) next hops. This
allows for manipulating multipath to be non-ECMP-based or
forwarding-based on application-defined paths, through
establishment of a peering session with an application
"controller" that can inject routing information into the system,
satisfying REQ5. OSPF provides similar functionality using
concepts such as "Forwarding Address", but with more difficulty in
implementation and far less control of information propagation
scope.
o Using a well-defined Autonomous System Number (ASN) allocation
scheme and standard AS_PATH loop detection, "BGP path hunting"
(see [JAKMA2008]) can be controlled and complex unwanted paths
will be ignored. See Section 5.2 for an example of a working ASN
allocation scheme. In a link-state IGP, accomplishing the same
goal would require multi-(instance/topology/process) support,
typically not available in all DC devices and quite complex to
configure and troubleshoot. Using a traditional single flooding
domain, which most DC designs utilize, under certain failure
conditions may pick up unwanted lengthy paths, e.g., traversing
multiple Tier 2 devices.
o EBGP configuration that is implemented with minimal routing policy
is easier to troubleshoot for network reachability issues. In
most implementations, it is straightforward to view contents of
the BGP Loc-RIB and compare it to the router's Routing Information
Base (RIB). Also, in most implementations, an operator can view
every BGP neighbors Adj-RIB-In and Adj-RIB-Out structures, and
therefore incoming and outgoing Network Layer Reachability
Information (NLRI) information can be easily correlated on both
sides of a BGP session. Thus, BGP satisfies REQ3.
5.2. EBGP Configuration for Clos Topology
Clos topologies that have more than 5 stages are very uncommon due to
the large numbers of interconnects required by such a design.
Therefore, the examples below are made with reference to the 5-stage
Clos topology (in unfolded state).
5.2.1. EBGP Configuration Guidelines and Example ASN Scheme
The diagram below illustrates an example of an ASN allocation scheme.
The following is a list of guidelines that can be used:
o EBGP single-hop sessions are established over direct point-to-
point links interconnecting the network nodes, no multi-hop or
loopback sessions are used, even in the case of multiple links
between the same pair of nodes.
o Private Use ASNs from the range 64512-65534 are used to avoid ASN
conflicts.
o A single ASN is allocated to all of the Clos topology's Tier 1
devices.
o A unique ASN is allocated to each set of Tier 2 devices in the
same cluster.
o A unique ASN is allocated to every Tier 3 device (e.g., ToR) in
this topology.
ASN 65534
+---------+
| +-----+ |
| | | |
+-|-| |-|-+
| | +-----+ | |
ASN 646XX | | | | ASN 646XX
+---------+ | | | | +---------+
| +-----+ | | | +-----+ | | | +-----+ |
+-----------|-| |-|-+-|-| |-|-+-|-| |-|-----------+
| +---|-| |-|-+ | | | | +-|-| |-|---+ |
| | | +-----+ | | +-----+ | | +-----+ | | |
| | | | | | | | | |
| | | | | | | | | |
| | | +-----+ | | +-----+ | | +-----+ | | |
| +-----+---|-| |-|-+ | | | | +-|-| |-|---+-----+ |
| | | +-|-| |-|-+-|-| |-|-+-|-| |-|-+ | | |
| | | | | +-----+ | | | +-----+ | | | +-----+ | | | | |
| | | | +---------+ | | | | +---------+ | | | |
| | | | | | | | | | | |
+-----+ +-----+ | | +-----+ | | +-----+ +-----+
| ASN | | | +-|-| |-|-+ | | | |
|65YYY| | ... | | | | | | ... | | ... |
+-----+ +-----+ | +-----+ | +-----+ +-----+
| | | | +---------+ | | | |
O O O O <- Servers -> O O O O
Figure 4: BGP ASN Layout for 5-Stage Clos
5.2.2. Private Use ASNs
The original range of Private Use ASNs [RFC6996] limited operators to
1023 unique ASNs. Since it is quite likely that the number of
network devices may exceed this number, a workaround is required.
One approach is to re-use the ASNs assigned to the Tier 3 devices
across different clusters. For example, Private Use ASNs 65001,
65002 ... 65032 could be used within every individual cluster and
assigned to Tier 3 devices.
To avoid route suppression due to the AS_PATH loop detection
mechanism in BGP, upstream EBGP sessions on Tier 3 devices must be
configured with the "Allowas-in" feature [ALLOWASIN] that allows
accepting a device's own ASN in received route advertisements.
Although this feature is not standardized, it is widely available
across multiple vendors implementations. Introducing this feature
does not make routing loops more likely in the design since the
AS_PATH is being added to by routers at each of the topology tiers
and AS_PATH length is an early tie breaker in the BGP path selection
process. Further loop protection is still in place at the Tier 1
device, which will not accept routes with a path including its own
ASN. Tier 2 devices do not have direct connectivity with each other.
Another solution to this problem would be to use Four-Octet ASNs
([RFC6793]), where there are additional Private Use ASNs available,
see [IANA.AS]. Use of Four-Octet ASNs puts additional protocol
complexity in the BGP implementation and should be balanced against
the complexity of re-use when considering REQ3 and REQ4. Perhaps
more importantly, they are not yet supported by all BGP
implementations, which may limit vendor selection of DC equipment.
When supported, ensure that deployed implementations are able to
remove the Private Use ASNs when external connectivity
(Section 5.2.4) to these ASNs is required.
5.2.3. Prefix Advertisement
A Clos topology features a large number of point-to-point links and
associated prefixes. Advertising all of these routes into BGP may
create Forwarding Information Base (FIB) overload in the network
devices. Advertising these links also puts additional path
computation stress on the BGP control plane for little benefit.
There are two possible solutions:
o Do not advertise any of the point-to-point links into BGP. Since
the EBGP-based design changes the next-hop address at every
device, distant networks will automatically be reachable via the
advertising EBGP peer and do not require reachability to these
prefixes. However, this may complicate operations or monitoring:
e.g., using the popular "traceroute" tool will display IP
addresses that are not reachable.
o Advertise point-to-point links, but summarize them on every
device. This requires an address allocation scheme such as
allocating a consecutive block of IP addresses per Tier 1 and Tier
2 device to be used for point-to-point interface addressing to the
lower layers (Tier 2 uplinks will be allocated from Tier 1 address
blocks and so forth).
Server subnets on Tier 3 devices must be announced into BGP without
using route summarization on Tier 2 and Tier 1 devices. Summarizing
subnets in a Clos topology results in route black-holing under a
single link failure (e.g., between Tier 2 and Tier 3 devices), and
hence must be avoided. The use of peer links within the same tier to
resolve the black-holing problem by providing "bypass paths" is
undesirable due to O(N^2) complexity of the peering-mesh and waste of
ports on the devices. An alternative to the full mesh of peer links
would be to use a simpler bypass topology, e.g., a "ring" as
described in [FB4POST], but such a topology adds extra hops and has
limited bandwidth. It may require special tweaks to make BGP routing
work, e.g., splitting every device into an ASN of its own. Later in
this document, Section 8.2 introduces a less intrusive method for
performing a limited form of route summarization in Clos networks and
discusses its associated tradeoffs.
5.2.4. External Connectivity
A dedicated cluster (or clusters) in the Clos topology could be used
for the purpose of connecting to the Wide Area Network (WAN) edge
devices, or WAN Routers. Tier 3 devices in such a cluster would be
replaced with WAN routers, and EBGP peering would be used again,
though WAN routers are likely to belong to a public ASN if Internet
connectivity is required in the design. The Tier 2 devices in such a
dedicated cluster will be referred to as "Border Routers" in this
document. These devices have to perform a few special functions:
o Hide network topology information when advertising paths to WAN
routers, i.e., remove Private Use ASNs [RFC6996] from the AS_PATH
attribute. This is typically done to avoid ASN number collisions
between different data centers and also to provide a uniform
AS_PATH length to the WAN for purposes of WAN ECMP to anycast
prefixes originated in the topology. An implementation-specific
BGP feature typically called "Remove Private AS" is commonly used
to accomplish this. Depending on implementation, the feature
should strip a contiguous sequence of Private Use ASNs found in an
AS_PATH attribute prior to advertising the path to a neighbor.
This assumes that all ASNs used for intra data center numbering
are from the Private Use ranges. The process for stripping the
Private Use ASNs is not currently standardized, see [REMOVAL].
However, most implementations at least follow the logic described
in this vendor's document [VENDOR-REMOVE-PRIVATE-AS], which is
enough for the design specified.
o Originate a default route to the data center devices. This is the
only place where a default route can be originated, as route
summarization is risky for the unmodified Clos topology.
Alternatively, Border Routers may simply relay the default route
learned from WAN routers. Advertising the default route from
Border Routers requires that all Border Routers be fully connected
to the WAN Routers upstream, to provide resistance to a single-
link failure causing the black-holing of traffic. To prevent
black-holing in the situation when all of the EBGP sessions to the
WAN routers fail simultaneously on a given device, it is more
desirable to readvertise the default route rather than originating
the default route via complicated conditional route origination
schemes provided by some implementations [CONDITIONALROUTE].
5.2.5. Route Summarization at the Edge
It is often desirable to summarize network reachability information
prior to advertising it to the WAN network due to the high amount of
IP prefixes originated from within the data center in a fully routed
network design. For example, a network with 2000 Tier 3 devices will
have at least 2000 servers subnets advertised into BGP, along with
the infrastructure prefixes. However, as discussed in Section 5.2.3,
the proposed network design does not allow for route summarization
due to the lack of peer links inside every tier.
However, it is possible to lift this restriction for the Border
Routers by devising a different connectivity model for these devices.
There are two options possible:
o Interconnect the Border Routers using a full-mesh of physical
links or using any other "peer-mesh" topology, such as ring or
hub-and-spoke. Configure BGP accordingly on all Border Leafs to
exchange network reachability information, e.g., by adding a mesh
of IBGP sessions. The interconnecting peer links need to be
appropriately sized for traffic that will be present in the case
of a device or link failure in the mesh connecting the Border
Routers.
o Tier 1 devices may have additional physical links provisioned
toward the Border Routers (which are Tier 2 devices from the
perspective of Tier 1). Specifically, if protection from a single
link or node failure is desired, each Tier 1 device would have to
connect to at least two Border Routers. This puts additional
requirements on the port count for Tier 1 devices and Border
Routers, potentially making it a nonuniform, larger port count,
device compared with the other devices in the Clos. This also
reduces the number of ports available to "regular" Tier 2
switches, and hence the number of clusters that could be
interconnected via Tier 1.
If any of the above options are implemented, it is possible to
perform route summarization at the Border Routers toward the WAN
network core without risking a routing black-hole condition under a
single link failure. Both of the options would result in nonuniform
topology as additional links have to be provisioned on some network
devices.
6. ECMP Considerations
This section covers the Equal Cost Multipath (ECMP) functionality for
Clos topology and discusses a few special requirements.
6.1. Basic ECMP
ECMP is the fundamental load-sharing mechanism used by a Clos
topology. Effectively, every lower-tier device will use all of its
directly attached upper-tier devices to load-share traffic destined
to the same IP prefix. The number of ECMP paths between any two Tier
3 devices in Clos topology is equal to the number of the devices in
the middle stage (Tier 1). For example, Figure 5 illustrates a
topology where Tier 3 device A has four paths to reach servers X and
Y, via Tier 2 devices B and C and then Tier 1 devices 1, 2, 3, and 4,
respectively.
Tier 1
+-----+
| DEV |
+->| 1 |--+
| +-----+ |
Tier 2 | | Tier 2
+-----+ | +-----+ | +-----+
+------------>| DEV |--+->| DEV |--+--| |-------------+
| +-----| B |--+ | 2 | +--| |-----+ |
| | +-----+ +-----+ +-----+ | |
| | | |
| | +-----+ +-----+ +-----+ | |
| +-----+---->| DEV |--+ | DEV | +--| |-----+-----+ |
| | | +---| C |--+->| 3 |--+--| |---+ | | |
| | | | +-----+ | +-----+ | +-----+ | | | |
| | | | | | | | | |
+-----+ +-----+ | +-----+ | +-----+ +-----+
| DEV | | | Tier 3 +->| DEV |--+ Tier 3 | | | |
| A | | | | 4 | | | | |
+-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | | |
O O O O <- Servers -> X Y O O
Figure 5: ECMP Fan-Out Tree from A to X and Y
The ECMP requirement implies that the BGP implementation must support
multipath fan-out for up to the maximum number of devices directly
attached at any point in the topology in the upstream or downstream
direction. Normally, this number does not exceed half of the ports
found on a device in the topology. For example, an ECMP fan-out of
32 would be required when building a Clos network using 64-port
devices. The Border Routers may need to have wider fan-out to be
able to connect to a multitude of Tier 1 devices if route
summarization at Border Router level is implemented as described in
Section 5.2.5. If a device's hardware does not support wider ECMP,
logical link-grouping (link-aggregation at Layer 2) could be used to
provide "hierarchical" ECMP (Layer 3 ECMP coupled with Layer 2 ECMP)
to compensate for fan-out limitations. However, this approach
increases the risk of flow polarization, as less entropy will be
available at the second stage of ECMP.
Most BGP implementations declare paths to be equal from an ECMP
perspective if they match up to and including step (e) in
Section 9.1.2.2 of [RFC4271]. In the proposed network design there
is no underlying IGP, so all IGP costs are assumed to be zero or
otherwise the same value across all paths and policies may be applied
as necessary to equalize BGP attributes that vary in vendor defaults,
such as the MULTI_EXIT_DISC (MED) attribute and origin code. For
historical reasons, it is also useful to not use 0 as the equalized
MED value; this and some other useful BGP information is available in
[RFC4277]. Routing loops are unlikely due to the BGP best-path
selection process (which prefers shorter AS_PATH length), and longer
paths through the Tier 1 devices (which don't allow their own ASN in
the path) are not possible.
6.2. BGP ECMP over Multiple ASNs
For application load-balancing purposes, it is desirable to have the
same prefix advertised from multiple Tier 3 devices. From the
perspective of other devices, such a prefix would have BGP paths with
different AS_PATH attribute values, while having the same AS_PATH
attribute lengths. Therefore, BGP implementations must support load-
sharing over the above-mentioned paths. This feature is sometimes
known as "multipath relax" or "multipath multiple-AS" and effectively
allows for ECMP to be done across different neighboring ASNs if all
other attributes are equal as already described in the previous
section.
6.3. Weighted ECMP
It may be desirable for the network devices to implement "weighted"
ECMP, to be able to send more traffic over some paths in ECMP fan-
out. This could be helpful to compensate for failures in the network
and send more traffic over paths that have more capacity. The
prefixes that require weighted ECMP would have to be injected using
remote BGP speaker (central agent) over a multi-hop session as
described further in Section 8.1. If support in implementations is
available, weight distribution for multiple BGP paths could be
signaled using the technique described in [LINK].
6.4. Consistent Hashing
It is often desirable to have the hashing function used for ECMP to
be consistent (see [CONS-HASH]), to minimize the impact on flow to
next-hop affinity changes when a next hop is added or removed to an
ECMP group. This could be used if the network device is used as a
load balancer, mapping flows toward multiple destinations -- in this
case, losing or adding a destination will not have a detrimental
effect on currently established flows. One particular recommendation
on implementing consistent hashing is provided in [RFC2992], though
other implementations are possible. This functionality could be
naturally combined with weighted ECMP, with the impact of the next
hop changes being proportional to the weight of the given next hop.
The downside of consistent hashing is increased load on hardware
resource utilization, as typically more resources (e.g., Ternary
Content-Addressable Memory (TCAM) space) are required to implement a
consistent-hashing function.
7. Routing Convergence Properties
This section reviews routing convergence properties in the proposed
design. A case is made that sub-second convergence is achievable if
the implementation supports fast EBGP peering session deactivation
and timely RIB and FIB updates upon failure of the associated link.
7.1. Fault Detection Timing
BGP typically relies on an IGP to route around link/node failures
inside an AS, and implements either a polling-based or an event-
driven mechanism to obtain updates on IGP state changes. The
proposed routing design does not use an IGP, so the remaining
mechanisms that could be used for fault detection are BGP keep-alive
time-out (or any other type of keep-alive mechanism) and link-failure
triggers.
Relying solely on BGP keep-alive packets may result in high
convergence delays, on the order of multiple seconds (on many BGP
implementations the minimum configurable BGP hold timer value is
three seconds). However, many BGP implementations can shut down
local EBGP peering sessions in response to the "link down" event for
the outgoing interface used for BGP peering. This feature is
sometimes called "fast fallover". Since links in modern data centers
are predominantly point-to-point fiber connections, a physical
interface failure is often detected in milliseconds and subsequently
triggers a BGP reconvergence.
Ethernet links may support failure signaling or detection standards
such as Connectivity Fault Management (CFM) as described in
[IEEE8021Q]; this may make failure detection more robust.
Alternatively, some platforms may support Bidirectional Forwarding
Detection (BFD) [RFC5880] to allow for sub-second failure detection
and fault signaling to the BGP process. However, the use of either
of these presents additional requirements to vendor software and
possibly hardware, and may contradict REQ1. Until recently with
[RFC7130], BFD also did not allow detection of a single member link
failure on a LAG, which would have limited its usefulness in some
designs.
7.2. Event Propagation Timing
In the proposed design, the impact of the BGP
MinRouteAdvertisementIntervalTimer (MRAI timer), as specified in
Section 9.2.1.1 of [RFC4271], should be considered. Per the
standard, it is required for BGP implementations to space out
consecutive BGP UPDATE messages by at least MRAI seconds, which is
often a configurable value. The initial BGP UPDATE messages after an
event carrying withdrawn routes are commonly not affected by this
timer. The MRAI timer may present significant convergence delays
when a BGP speaker "waits" for the new path to be learned from its
peers and has no local backup path information.
In a Clos topology, each EBGP speaker typically has either one path
(Tier 2 devices don't accept paths from other Tier 2 in the same
cluster due to same ASN) or N paths for the same prefix, where N is a
significantly large number, e.g., N=32 (the ECMP fan-out to the next
tier). Therefore, if a link fails to another device from which a
path is received there is either no backup path at all (e.g., from
the perspective of a Tier 2 switch losing the link to a Tier 3
device), or the backup is readily available in BGP Loc-RIB (e.g.,
from the perspective of a Tier 2 device losing the link to a Tier 1
switch). In the former case, the BGP withdrawal announcement will
propagate without delay and trigger reconvergence on affected
devices. In the latter case, the best path will be re-evaluated, and
the local ECMP group corresponding to the new next-hop set will be
changed. If the BGP path was the best path selected previously, an
"implicit withdraw" will be sent via a BGP UPDATE message as
described as Option b in Section 3.1 of [RFC4271] due to the BGP
AS_PATH attribute changing.
7.3. Impact of Clos Topology Fan-Outs
Clos topology has large fan-outs, which may impact the "Up->Down"
convergence in some cases, as described in this section. In a
situation when a link between Tier 3 and Tier 2 device fails, the
Tier 2 device will send BGP UPDATE messages to all upstream Tier 1
devices, withdrawing the affected prefixes. The Tier 1 devices, in
turn, will relay these messages to all downstream Tier 2 devices
(except for the originator). Tier 2 devices other than the one
originating the UPDATE should then wait for ALL upstream Tier 1
devices to send an UPDATE message before removing the affected
prefixes and sending corresponding UPDATE downstream to connected
Tier 3 devices. If the original Tier 2 device or the relaying Tier 1
devices introduce some delay into their UPDATE message announcements,
the result could be UPDATE message "dispersion", that could be as
long as multiple seconds. In order to avoid such a behavior, BGP
implementations must support "update groups". The "update group" is
defined as a collection of neighbors sharing the same outbound policy
-- the local speaker will send BGP updates to the members of the
group synchronously.
The impact of such "dispersion" grows with the size of topology fan-
out and could also grow under network convergence churn. Some
operators may be tempted to introduce "route flap dampening" type
features that vendors include to reduce the control-plane impact of
rapidly flapping prefixes. However, due to issues described with
false positives in these implementations especially under such
"dispersion" events, it is not recommended to enable this feature in
this design. More background and issues with "route flap dampening"
and possible implementation changes that could affect this are well
described in [RFC7196].
7.4. Failure Impact Scope
A network is declared to converge in response to a failure once all
devices within the failure impact scope are notified of the event and
have recalculated their RIBs and consequently updated their FIBs.
Larger failure impact scope typically means slower convergence since
more devices have to be notified, and results in a less stable
network. In this section, we describe BGP's advantages over link-
state routing protocols in reducing failure impact scope for a Clos
topology.
BGP behaves like a distance-vector protocol in the sense that only
the best path from the point of view of the local router is sent to
neighbors. As such, some failures are masked if the local node can
immediately find a backup path and does not have to send any updates
further. Notice that in the worst case, all devices in a data center
topology have to either withdraw a prefix completely or update the
ECMP groups in their FIBs. However, many failures will not result in
such a wide impact. There are two main failure types where impact
scope is reduced:
o Failure of a link between Tier 2 and Tier 1 devices: In this case,
a Tier 2 device will update the affected ECMP groups, removing the
failed link. There is no need to send new information to
downstream Tier 3 devices, unless the path was selected as best by
the BGP process, in which case only an "implicit withdraw" needs
to be sent and this should not affect forwarding. The affected
Tier 1 device will lose the only path available to reach a
particular cluster and will have to withdraw the associated
prefixes. Such a prefix withdrawal process will only affect Tier
2 devices directly connected to the affected Tier 1 device. The
Tier 2 devices receiving the BGP UPDATE messages withdrawing
prefixes will simply have to update their ECMP groups. The Tier 3
devices are not involved in the reconvergence process.
o Failure of a Tier 1 device: In this case, all Tier 2 devices
directly attached to the failed node will have to update their
ECMP groups for all IP prefixes from a non-local cluster. The
Tier 3 devices are once again not involved in the reconvergence
process, but may receive "implicit withdraws" as described above.
Even in the case of such failures where multiple IP prefixes will
have to be reprogrammed in the FIB, it is worth noting that all of
these prefixes share a single ECMP group on a Tier 2 device.
Therefore, in the case of implementations with a hierarchical FIB,
only a single change has to be made to the FIB. "Hierarchical FIB"
here means FIB structure where the next-hop forwarding information is
stored separately from the prefix lookup table, and the latter only
stores pointers to the respective forwarding information. See
[BGP-PIC] for discussion of FIB hierarchies and fast convergence.
Even though BGP offers reduced failure scope for some cases, further
reduction of the fault domain using summarization is not always
possible with the proposed design, since using this technique may
create routing black-holes as mentioned previously. Therefore, the
worst failure impact scope on the control plane is the network as a
whole -- for instance, in the case of a link failure between Tier 2
and Tier 3 devices. The amount of impacted prefixes in this case
would be much less than in the case of a failure in the upper layers
of a Clos network topology. The property of having such large
failure scope is not a result of choosing EBGP in the design but
rather a result of using the Clos topology.
7.5. Routing Micro-Loops
When a downstream device, e.g., Tier 2 device, loses all paths for a
prefix, it normally has the default route pointing toward the
upstream device -- in this case, the Tier 1 device. As a result, it
is possible to get in the situation where a Tier 2 switch loses a
prefix, but a Tier 1 switch still has the path pointing to the Tier 2
device; this results in a transient micro-loop, since the Tier 1
switch will keep passing packets to the affected prefix back to the
Tier 2 device, and the Tier 2 will bounce them back again using the
default route. This micro-loop will last for the time it takes the
upstream device to fully update its forwarding tables.
To minimize impact of such micro-loops, Tier 2 and Tier 1 switches
can be configured with static "discard" or "null" routes that will be
more specific than the default route for prefixes missing during
network convergence. For Tier 2 switches, the discard route should
be a summary route, covering all server subnets of the underlying
Tier 3 devices. For Tier 1 devices, the discard route should be a
summary covering the server IP address subnets allocated for the
whole data center. Those discard routes will only take precedence
for the duration of network convergence, until the device learns a
more specific prefix via a new path.
8. Additional Options for Design
8.1. Third-Party Route Injection
BGP allows for a "third-party", i.e., a directly attached BGP
speaker, to inject routes anywhere in the network topology, meeting
REQ5. This can be achieved by peering via a multi-hop BGP session
with some or even all devices in the topology. Furthermore, BGP
diverse path distribution [RFC6774] could be used to inject multiple
BGP next hops for the same prefix to facilitate load balancing, or
using the BGP ADD-PATH capability [RFC7911] if supported by the
implementation. Unfortunately, in many implementations, ADD-PATH has
been found to only support IBGP properly in the use cases for which
it was originally optimized; this limits the "third-party" peering to
IBGP only.
To implement route injection in the proposed design, a third-party
BGP speaker may peer with Tier 3 and Tier 1 switches, injecting the
same prefix, but using a special set of BGP next hops for Tier 1
devices. Those next hops are assumed to resolve recursively via BGP,
and could be, for example, IP addresses on Tier 3 devices. The
resulting forwarding table programming could provide desired traffic
proportion distribution among different clusters.
8.2. Route Summarization within Clos Topology
As mentioned previously, route summarization is not possible within
the proposed Clos topology since it makes the network susceptible to
route black-holing under single link failures. The main problem is
the limited number of redundant paths between network elements, e.g.,
there is only a single path between any pair of Tier 1 and Tier 3
devices. However, some operators may find route aggregation
desirable to improve control-plane stability.
If any technique to summarize within the topology is planned,
modeling of the routing behavior and potential for black-holing
should be done not only for single or multiple link failures, but
also for fiber pathway failures or optical domain failures when the
topology extends beyond a physical location. Simple modeling can be
done by checking the reachability on devices doing summarization
under the condition of a link or pathway failure between a set of
devices in every tier as well as to the WAN routers when external
connectivity is present.
Route summarization would be possible with a small modification to
the network topology, though the tradeoff would be reduction of the
total size of the network as well as network congestion under
specific failures. This approach is very similar to the technique
described above, which allows Border Routers to summarize the entire
data center address space.
8.2.1. Collapsing Tier 1 Devices Layer
In order to add more paths between Tier 1 and Tier 3 devices, group
Tier 2 devices into pairs, and then connect the pairs to the same
group of Tier 1 devices. This is logically equivalent to
"collapsing" Tier 1 devices into a group of half the size, merging
the links on the "collapsed" devices. The result is illustrated in
Figure 6. For example, in this topology DEV C and DEV D connect to
the same set of Tier 1 devices (DEV 1 and DEV 2), whereas before they
were connecting to different groups of Tier 1 devices.
Tier 2 Tier 1 Tier 2
+-----+ +-----+ +-----+
+-------------| DEV |------| DEV |------| |-------------+
| +-----| C |--++--| 1 |--++--| |-----+ |
| | +-----+ || +-----+ || +-----+ | |
| | || || | |
| | +-----+ || +-----+ || +-----+ | |
| +-----+-----| DEV |--++--| DEV |--++--| |-----+-----+ |
| | | +---| D |------| 2 |------| |---+ | | |
| | | | +-----+ +-----+ +-----+ | | | |
| | | | | | | |
+-----+ +-----+ +-----+ +-----+
| DEV | | DEV | | | | |
| A | | B | Tier 3 Tier 3 | | | |
+-----+ +-----+ +-----+ +-----+
| | | | | | | |
O O O O <- Servers -> O O O O
Figure 6: 5-Stage Clos Topology
Having this design in place, Tier 2 devices may be configured to
advertise only a default route down to Tier 3 devices. If a link
between Tier 2 and Tier 3 fails, the traffic will be re-routed via
the second available path known to a Tier 2 switch. It is still not
possible to advertise a summary route covering prefixes for a single
cluster from Tier 2 devices since each of them has only a single path
down to this prefix. It would require dual-homed servers to
accomplish that. Also note that this design is only resilient to
single link failures. It is possible for a double link failure to
isolate a Tier 2 device from all paths toward a specific Tier 3
device, thus causing a routing black-hole.
A result of the proposed topology modification would be a reduction
of the port capacity of Tier 1 devices. This limits the maximum
number of attached Tier 2 devices, and therefore will limit the
maximum DC network size. A larger network would require different
Tier 1 devices that have higher port density to implement this
change.
Another problem is traffic rebalancing under link failures. Since
there are two paths from Tier 1 to Tier 3, a failure of the link
between Tier 1 and Tier 2 switch would result in all traffic that was
taking the failed link to switch to the remaining path. This will
result in doubling the link utilization on the remaining link.
8.2.2. Simple Virtual Aggregation
A completely different approach to route summarization is possible,
provided that the main goal is to reduce the FIB size, while allowing
the control plane to disseminate full routing information. Firstly,
it could be easily noted that in many cases multiple prefixes, some
of which are less specific, share the same set of the next hops (same
ECMP group). For example, from the perspective of Tier 3 devices,
all routes learned from upstream Tier 2 devices, including the
default route, will share the same set of BGP next hops, provided
that there are no failures in the network. This makes it possible to
use the technique similar to that described in [RFC6769] and only
install the least specific route in the FIB, ignoring more specific
routes if they share the same next-hop set. For example, under
normal network conditions, only the default route needs to be
programmed into the FIB.
Furthermore, if the Tier 2 devices are configured with summary
prefixes covering all of their attached Tier 3 device's prefixes, the
same logic could be applied in Tier 1 devices as well and, by
induction to Tier 2/Tier 3 switches in different clusters. These
summary routes should still allow for more specific prefixes to leak
to Tier 1 devices, to enable detection of mismatches in the next-hop
sets if a particular link fails, thus changing the next-hop set for a
specific prefix.
Restating once again, this technique does not reduce the amount of
control-plane state (i.e., BGP UPDATEs, BGP Loc-RIB size), but only
allows for more efficient FIB utilization, by detecting more specific
prefixes that share their next-hop set with a subsuming less specific
prefix.
8.3. ICMP Unreachable Message Masquerading
This section discusses some operational aspects of not advertising
point-to-point link subnets into BGP, as previously identified as an
option in Section 5.2.3. The operational impact of this decision
could be seen when using the well-known "traceroute" tool.
Specifically, IP addresses displayed by the tool will be the link's
point-to-point addresses, and hence will be unreachable for
management connectivity. This makes some troubleshooting more
complicated.
One way to overcome this limitation is by using the DNS subsystem to
create the "reverse" entries for these point-to-point IP addresses
pointing to the same name as the loopback address. The connectivity
then can be made by resolving this name to the "primary" IP address
of the device, e.g., its Loopback interface, which is always
advertised into BGP. However, this creates a dependency on the DNS
subsystem, which may be unavailable during an outage.
Another option is to make the network device perform IP address
masquerading, that is, rewriting the source IP addresses of the
appropriate ICMP messages sent by the device with the "primary" IP
address of the device. Specifically, the ICMP Destination
Unreachable Message (type 3) code 3 (port unreachable) and ICMP Time
Exceeded (type 11) code 0 are required for correct operation of the
"traceroute" tool. With this modification, the "traceroute" probes
sent to the devices will always be sent back with the "primary" IP
address as the source, allowing the operator to discover the
"reachable" IP address of the box. This has the downside of hiding
the address of the "entry point" into the device. If the devices
support [RFC5837], this may allow the best of both worlds by
providing the information about the incoming interface even if the
return address is the "primary" IP address.
9. Security Considerations
The design does not introduce any additional security concerns.
General BGP security considerations are discussed in [RFC4271] and
[RFC4272]. Since a DC is a single-operator domain, this document
assumes that edge filtering is in place to prevent attacks against
the BGP sessions themselves from outside the perimeter of the DC.
This may be a more feasible option for most deployments than having
to deal with key management for TCP MD5 as described in [RFC2385] or
dealing with the lack of implementations of the TCP Authentication
Option [RFC5925] available at the time of publication of this
document. The Generalized TTL Security Mechanism [RFC5082] could
also be used to further reduce the risk of BGP session spoofing.
10. References
10.1. Normative References
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<http://www.rfc-editor.org/info/rfc4271>.
[RFC6996] Mitchell, J., "Autonomous System (AS) Reservation for
Private Use", BCP 6, RFC 6996, DOI 10.17487/RFC6996, July
2013, <http://www.rfc-editor.org/info/rfc6996>.
10.2. Informative References
[ALFARES2008]
Al-Fares, M., Loukissas, A., and A. Vahdat, "A Scalable,
Commodity Data Center Network Architecture",
DOI 10.1145/1402958.1402967, August 2008,
<http://dl.acm.org/citation.cfm?id=1402967>.
[ALLOWASIN]
Cisco Systems, "Allowas-in Feature in BGP Configuration
Example", February 2015,
<http://www.cisco.com/c/en/us/support/docs/ip/
border-gateway-protocol-bgp/112236-allowas-in-bgp-config-
example.html>.
[BGP-PIC] Bashandy, A., Ed., Filsfils, C., and P. Mohapatra, "BGP
Prefix Independent Convergence", Work in Progress,
draft-ietf-rtgwg-bgp-pic-02, August 2016.
[CLOS1953] Clos, C., "A Study of Non-Blocking Switching Networks",
The Bell System Technical Journal, Vol. 32(2),
DOI 10.1002/j.1538-7305.1953.tb01433.x, March 1953.
[CONDITIONALROUTE]
Cisco Systems, "Configuring and Verifying the BGP
Conditional Advertisement Feature", August 2005,
<http://www.cisco.com/c/en/us/support/docs/ip/
border-gateway-protocol-bgp/16137-cond-adv.html>.
[CONS-HASH]
Wikipedia, "Consistent Hashing", July 2016,
<https://en.wikipedia.org/w/
index.php?title=Consistent_hashing&oldid=728825684>.
[FB4POST] Farrington, N. and A. Andreyev, "Facebook's Data Center
Network Architecture", May 2013,
<http://nathanfarrington.com/papers/facebook-oic13.pdf>.
[GREENBERG2009]
Greenberg, A., Hamilton, J., and D. Maltz, "The Cost of a
Cloud: Research Problems in Data Center Networks",
DOI 10.1145/1496091.1496103, January 2009,
<http://dl.acm.org/citation.cfm?id=1496103>.
[HADOOP] Apache, "Apache Hadoop", April 2016,
<https://hadoop.apache.org/>.
[IANA.AS] IANA, "Autonomous System (AS) Numbers",
<http://www.iana.org/assignments/as-numbers>.
[IEEE8021D-1990]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks: Media Access Control (MAC) Bridges", IEEE
Std 802.1D, DOI 10.1109/IEEESTD.1991.101050, 1991,
<http://ieeexplore.ieee.org/servlet/opac?punumber=2255>.
[IEEE8021D-2004]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks: Media Access Control (MAC) Bridges", IEEE
Std 802.1D, DOI 10.1109/IEEESTD.2004.94569, June 2004,
<http://ieeexplore.ieee.org/servlet/opac?punumber=9155>.
[IEEE8021Q]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks: Bridges and Bridged Networks", IEEE Std 802.1Q,
DOI 10.1109/IEEESTD.2014.6991462,
<http://ieeexplore.ieee.org/servlet/
opac?punumber=6991460>.
[IEEE8023AD]
IEEE, "Amendment to Carrier Sense Multiple Access With
Collision Detection (CSMA/CD) Access Method and Physical
Layer Specifications - Aggregation of Multiple Link
Segments", IEEE Std 802.3ad,
DOI 10.1109/IEEESTD.2000.91610, October 2000,
<http://ieeexplore.ieee.org/servlet/opac?punumber=6867>.
[INTERCON] Dally, W. and B. Towles, "Principles and Practices of
Interconnection Networks", ISBN 978-0122007514, January
2004, <http://dl.acm.org/citation.cfm?id=995703>.
[JAKMA2008]
Jakma, P., "BGP Path Hunting", 2008,
<https://blogs.oracle.com/paulj/entry/bgp_path_hunting>.
[L3DSR] Schaumann, J., "L3DSR - Overcoming Layer 2 Limitations of
Direct Server Return Load Balancing", 2011,
<https://www.nanog.org/meetings/nanog51/presentations/
Monday/NANOG51.Talk45.nanog51-Schaumann.pdf>.
[LINK] Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
Extended Community", Work in Progress, draft-ietf-idr-
link-bandwidth-06, January 2013.
[REMOVAL] Mitchell, J., Rao, D., and R. Raszuk, "Private Autonomous
System (AS) Removal Requirements", Work in Progress,
draft-mitchell-grow-remove-private-as-04, April 2015.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
<http://www.rfc-editor.org/info/rfc2328>.
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, DOI 10.17487/RFC2385, August
1998, <http://www.rfc-editor.org/info/rfc2385>.
[RFC2992] Hopps, C., "Analysis of an Equal-Cost Multi-Path
Algorithm", RFC 2992, DOI 10.17487/RFC2992, November 2000,
<http://www.rfc-editor.org/info/rfc2992>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<http://www.rfc-editor.org/info/rfc4272>.
[RFC4277] McPherson, D. and K. Patel, "Experience with the BGP-4
Protocol", RFC 4277, DOI 10.17487/RFC4277, January 2006,
<http://www.rfc-editor.org/info/rfc4277>.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <http://www.rfc-editor.org/info/rfc4786>.
[RFC5082] Gill, V., Heasley, J., Meyer, D., Savola, P., Ed., and C.
Pignataro, "The Generalized TTL Security Mechanism
(GTSM)", RFC 5082, DOI 10.17487/RFC5082, October 2007,
<http://www.rfc-editor.org/info/rfc5082>.
[RFC5837] Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
N., and JR. Rivers, "Extending ICMP for Interface and
Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
April 2010, <http://www.rfc-editor.org/info/rfc5837>.
[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
<http://www.rfc-editor.org/info/rfc5880>.
[RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP
Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
June 2010, <http://www.rfc-editor.org/info/rfc5925>.
[RFC6325] Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
Ghanwani, "Routing Bridges (RBridges): Base Protocol
Specification", RFC 6325, DOI 10.17487/RFC6325, July 2011,
<http://www.rfc-editor.org/info/rfc6325>.
[RFC6769] Raszuk, R., Heitz, J., Lo, A., Zhang, L., and X. Xu,
"Simple Virtual Aggregation (S-VA)", RFC 6769,
DOI 10.17487/RFC6769, October 2012,
<http://www.rfc-editor.org/info/rfc6769>.
[RFC6774] Raszuk, R., Ed., Fernando, R., Patel, K., McPherson, D.,
and K. Kumaki, "Distribution of Diverse BGP Paths",
RFC 6774, DOI 10.17487/RFC6774, November 2012,
<http://www.rfc-editor.org/info/rfc6774>.
[RFC6793] Vohra, Q. and E. Chen, "BGP Support for Four-Octet
Autonomous System (AS) Number Space", RFC 6793,
DOI 10.17487/RFC6793, December 2012,
<http://www.rfc-editor.org/info/rfc6793>.
[RFC7067] Dunbar, L., Eastlake 3rd, D., Perlman, R., and I.
Gashinsky, "Directory Assistance Problem and High-Level
Design Proposal", RFC 7067, DOI 10.17487/RFC7067, November
2013, <http://www.rfc-editor.org/info/rfc7067>.
[RFC7130] Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
Forwarding Detection (BFD) on Link Aggregation Group (LAG)
Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
2014, <http://www.rfc-editor.org/info/rfc7130>.
[RFC7196] Pelsser, C., Bush, R., Patel, K., Mohapatra, P., and O.
Maennel, "Making Route Flap Damping Usable", RFC 7196,
DOI 10.17487/RFC7196, May 2014,
<http://www.rfc-editor.org/info/rfc7196>.
[RFC7911] Walton, D., Retana, A., Chen, E., and J. Scudder,
"Advertisement of Multiple Paths in BGP", RFC 7911,
DOI 10.17487/RFC7911, July 2016,
<http://www.rfc-editor.org/info/rfc7911>.
[VENDOR-REMOVE-PRIVATE-AS]
Cisco Systems, "Removing Private Autonomous System Numbers
in BGP", August 2005,
<http://www.cisco.com/en/US/tech/tk365/
technologies_tech_note09186a0080093f27.shtml>.
Acknowledgements
This publication summarizes the work of many people who participated
in developing, testing, and deploying the proposed network design,
some of whom were George Chen, Parantap Lahiri, Dave Maltz, Edet
Nkposong, Robert Toomey, and Lihua Yuan. The authors would also like
to thank Linda Dunbar, Anoop Ghanwani, Susan Hares, Danny McPherson,
Robert Raszuk, and Russ White for reviewing this document and
providing valuable feedback, and Mary Mitchell for initial grammar
and style suggestions.
Authors' Addresses
Petr Lapukhov
Facebook
1 Hacker Way
Menlo Park, CA 94025
United States of America
Email: petr@fb.com
Ariff Premji
Arista Networks
5453 Great America Parkway
Santa Clara, CA 95054
United States of America
Email: ariff@arista.com
URI: http://arista.com/
Jon Mitchell (editor)
Email: jrmitche@puck.nether.net
ISO Plain Language
On this page
§1What the Document Is About
The document describes a practical routing design for use in a large-scale data centre. Such data centres, also known as hyper-scale or warehouse-scale, have the distinctive attribute of supporting over one hundred thousand servers, and operators are revisiting networking designs and platforms to accommodate networks of that scale.
The design is based on operational experience with data centres built to support large-scale distributed software infrastructure, such as a web search engine. The primary requirements in such an environment are operational simplicity and network stability, so that a small group of people can effectively support a significantly sized network.
Experimentation and extensive testing have shown that External BGP, defined in RFC 4271, is well suited as a stand-alone routing protocol for these types of data centre application. This contrasts with more traditional data centre designs, which may use simple tree topologies and rely on extending Layer 2 domains across multiple network devices. The document sets out the requirements that led to the choice, presents the details of the EBGP routing design, and explores ideas for further enhancements. A thorough understanding of BGP is assumed of a reader planning to deploy the design.
The requirements themselves follow from the traffic the network must carry. Until recently it was common for most traffic to enter and leave the data centre, called north-south traffic, and traditional tree topologies with high oversubscription ratios between layers were sufficient. Today many large-scale data centres host applications generating significant amounts of server-to-server traffic that does not leave the data centre, called east-west traffic, such as computer clusters like Hadoop, massive data replication between clusters, or virtual machine migrations. Scaling tree topologies to match these bandwidth demands becomes either too expensive or impossible because of physical limitations such as port density in a switch.
Cost shapes the rest of the design. The capital expenditure on network infrastructure constitutes about 10-15% of total data centre expenditure, so costs are driven down by unifying all network elements, preferably with the same hardware type or even the same device, which allows volume pricing and reduces maintenance and inventory costs, and by introducing multiple equipment vendors, which requires minimising the software feature requirements so that interoperability rests on open standards. Operating large-scale infrastructure is expensive because a larger number of elements fail more often, and a simpler design using a limited software feature set minimises software-related failures. Ethernet networks are susceptible to broadcast or unicast storms with a dramatic effect on performance and availability, and a fully routed design significantly reduces the size of the data-plane failure domains, limiting them to the lowest level of the hierarchy, though this introduces distributed control-plane failures, which argues for simpler control-plane protocols and fewer protocol interaction issues.
Big data centres need a network that grows by adding more of the same switches, does not break in one place, and can be run by a handful of people. The document says that running the Internet's own routing protocol inside the building achieves that.
§2.5The Five Design Requirements
Section 2.5 summarises the requirements the design has to meet. REQ1 asks for a topology that can be scaled horizontally by adding more links and network devices of the same type, without requiring upgrades to the network elements themselves. REQ2 asks for a narrow set of software features and protocols supported by a multitude of networking equipment vendors. REQ3 asks for a routing protocol with a simple implementation in terms of programming code complexity and ease of operational support. REQ4 asks to minimise the failure domain of equipment or protocol issues as much as possible. REQ5 asks to allow some traffic engineering, preferably through explicit control of the routing prefix next hop using built-in protocol mechanics.
Everything else in the document follows from five rules: grow sideways, use standard features many vendors support, keep the protocol simple, keep failures small, and allow traffic to be steered deliberately.
- REQ1A topology that scales horizontally by adding more links and devices of the same type, without upgrading the elements themselves.
- REQ2A narrow set of software features and protocols supported by a multitude of networking equipment vendors.
- REQ3A routing protocol with a simple implementation in code complexity and ease of operational support.
- REQ4Minimise the failure domain of equipment or protocol issues as much as possible.
- REQ5Allow some traffic engineering, preferably through explicit control of the routing prefix next hop using built-in protocol mechanics.
§3Tree Topologies, Clos Topologies and the Layer 3 Choice
The traditional data centre topology looks like an upside-down tree with redundant uplinks and three layers of hierarchy, which the document calls Tier 1, Tier 2 and Tier 3, using core, aggregation and access for comparison. Each higher layer has higher port density and bandwidth capacity, with the core as the trunk. It cannot be scaled to a large enough degree, because Tier 1 devices with enough port density to scale Tier 2 cannot be acquired, and continuous upgrades or replacement of the upper tiers are required as deployment size or bandwidth grows, which is operationally complex. That is why REQ1 exists and why this design is ruled out.
The alternative is a folded Clos topology, sometimes called a fat tree, which features an odd number of stages and is commonly made of uniform elements such as switches with the same port count. This is also called a leaf and spine network, where spine names the middle stage and leaf the input/output stage. Its key properties are that it is fully non-blocking, or more accurately non-interfering, if M is at least N and is oversubscribed by a factor of N/M otherwise, where M and N are the uplink and downlink port counts of a Tier 2 switch; that using it requires control and data-plane support for ECMP with a fan-out of M or more; that Tier 1 switches have exactly one path to every server, which makes route summarisation dangerous; and that traffic flowing from server to server is load balanced over all available paths using ECMP. A Clos topology can be scaled either by increasing network element port density or by adding more stages, for example moving to a five-stage Clos, and a set of directly connected Tier 2 and Tier 3 devices along with their attached servers is called a cluster, a useful unit of deployment or maintenance. Tier 3, typically top-of-rack switches, is where oversubscription is introduced, and limiting oversubscription to a single layer simplifies application development that would otherwise have to account for multiple bandwidth pools. If the network is small, the number of switches in Tier 1 or Tier 2 can be reduced by a factor of two by mapping two uplinks from a Tier 2 device to the same Tier 1 device, which saves cost but halves the maximum data centre size and requires the parallel links to be grouped in link aggregation groups, or an equivalent fate-sharing technique, so that one failed link does not overload the other.
On the routing question there are three designs. Layer 2 only designs originally used Spanning Tree Protocol for loop-free topology creation. RSTP and MST improve convergence and load balancing but the fundamentals of the protocol limit its applicability: STP and its variants use an active/standby approach to path selection and are therefore hard to deploy in horizontally scaled topologies, and operators have had many experiences of large failures caused by improper cabling, misconfiguration or flawed software on a single device, regularly affecting the entire spanning-tree domain and being very hard to troubleshoot. Multi-Chassis Link Aggregation made it possible to use active-active paths with STP as the backup, but the approach does not scale linearly past two in most implementations, lacks standards-based implementations and adds the failure domain risk of syncing state between devices. TRILL allows large horizontally scalable Layer 2-only networks without STP and resolves many of STP's problems, but the limited number of implementations and the requirement for specific equipment have limited its applicability and raised its cost, and neither TRILL nor M-LAG eliminates the shared broadcast domain, so TRILL-based designs have issues meeting REQ2, REQ3 and REQ4. Hybrid Layer 2 and Layer 3 designs put a routing protocol in either Tier 1 or Tier 2 and divide the Layer 2 domain into numerous smaller domains, which allows scaling but costs complexity in managing multiple network protocols; operators retain Layer 2 in the access tier, or in both access and aggregation, to support legacy applications needing direct Layer 2 adjacency or non-IP protocols, to give virtual machines seamless mobility with preserved IP addresses, to simplify IP addressing, to allow application load balancing techniques that need direct Layer 2 reachability, and because of continued cost differences between Layer 2- and Layer 3-capable switches. Layer 3 only designs, which route down to Tier 3, gain improved stability and scalability by confining Layer 2 broadcast domains, and are typically built on an interior gateway protocol such as OSPF. As data centres grow past tens of thousands of servers, fully routed designs become more attractive, and choosing a Layer 3-only design greatly simplifies the network and helps meet REQ1 and REQ2.
The old shape was a tree with three layers, which cannot grow far enough. The replacement is a Clos, a set of identical switches arranged so every server has several paths to every other server. The document also rules out designs that keep large Layer 2 domains, because one faulty switch or cable can take out a whole floor of the building.
- Clos topology
- Tier 1, spine A single ASN is allocated to all Tier 1 devices.
- Tier 2, leaf A unique ASN for each set of Tier 2 devices in the same cluster.
- Tier 3, top-of-rack A unique ASN for every Tier 3 device.
- Tier 2, leaf A unique ASN for each set of Tier 2 devices in the same cluster.
- Servers Reached through Tier 3; every server has several paths to every other server.
- Tier 1, spine A single ASN is allocated to all Tier 1 devices.
| Design | Routing | Assessment in the document |
|---|---|---|
| Layer 2 only | Spanning Tree Protocol, with RSTP, MST or TRILL as variants. | Active and standby path selection does not scale horizontally, and a single cabling, configuration or software fault can affect a whole spanning-tree domain. |
| Hybrid Layer 2/L3 | A routing protocol in Tier 1 or Tier 2, with many small Layer 2 domains. | Scales up, but at the cost of running several protocols; Layer 2 is kept for legacy applications, virtual machine mobility, simpler addressing and load balancing. |
| Layer 3 only | IP routing down to Tier 3, usually with an interior gateway protocol such as OSPF. | Confines Layer 2 broadcast domains, improves stability and scalability, and simplifies the network, helping to meet REQ1 and REQ2. |
§5Running BGP as the Only Routing Protocol
REQ2 favours a single routing protocol to reduce complexity and interdependencies. While it is common to rely on an IGP, sometimes with EBGP added at the device bordering the WAN or IBGP throughout, the document proposes an EBGP-only design. BGP is not generally deployed as the primary routing protocol inside a data centre because it is perceived as a WAN-only protocol, because it is believed to converge much more slowly than IGPs, because large-scale BGP deployments usually use an IGP for next-hop resolution, and because it is believed to require significant configuration overhead and to lack neighbour auto-discovery.
The document answers each of those perceptions. BGP has less complexity in parts of its design, since its internal data structures and state machine are simpler than those of most link-state IGPs such as OSPF: instead of implementing adjacency formation, adjacency maintenance and flow control, it relies on TCP as the underlying transport, which serves REQ2 and REQ3. Its information flooding overhead is lower, because every BGP router calculates and propagates only the selected best path, so a network failure is masked as soon as the speaker finds an alternate path, which exists when highly symmetric topologies such as Clos are combined with an EBGP-only design; the event propagation scope of a link-state IGP, by contrast, is an entire area regardless of failure type, so BGP better meets REQ3 and REQ4. BGP supports third-party, recursively resolved next hops, which allows multipath to be non-ECMP-based or forwarding-based on application-defined paths by establishing a peering session with an application controller that injects routing information, satisfying REQ5; OSPF offers similar functionality through concepts such as the Forwarding Address but with more implementation difficulty and far less control of information propagation scope. With a well-defined ASN allocation scheme and standard AS_PATH loop detection, BGP path hunting can be controlled and complex unwanted paths ignored, whereas in a link-state IGP the same goal would need multi-instance, multi-topology or multi-process support that is not available on all data centre devices and is complex to configure and troubleshoot. Finally, EBGP configuration with minimal routing policy is easier to troubleshoot, because in most implementations it is straightforward to view the contents of the BGP Loc-RIB and compare them with the router's RIB, and an operator can view every neighbour's Adj-RIB-In and Adj-RIB-Out structures, so incoming and outgoing NLRI can be correlated on both sides of a session.
The configuration guidelines for a Clos topology are explicit. EBGP single-hop sessions are established over direct point-to-point links, with no multi-hop or loopback sessions even where multiple links join the same pair of nodes. Private Use ASNs from the range 64512-65534 are used to avoid ASN conflicts. A single ASN is allocated to all Tier 1 devices, a unique ASN to each set of Tier 2 devices in the same cluster, and a unique ASN to every Tier 3 device. The original Private Use ASN range limited operators to 1023 unique ASNs, so one workaround is to re-use the ASNs assigned to Tier 3 devices across different clusters, for example 65001, 65002 and so on up to 65032 within each cluster; to avoid route suppression from AS_PATH loop detection, the upstream EBGP sessions on Tier 3 devices must be configured with the Allowas-in feature, which is not standardised but widely available, and which does not make routing loops more likely because the AS_PATH is added to at each tier and its length is an early tie breaker in the BGP path selection process, with further loop protection still in place at the Tier 1 device, which will not accept routes whose path includes its own ASN. Four-Octet ASNs are an alternative solution, but they add protocol complexity and are not yet supported by all BGP implementations, which may limit vendor selection.
On prefix advertisement, a Clos topology has a large number of point-to-point links and associated prefixes, and advertising all of them may overload the FIB and stress the control plane for little benefit. The document offers two solutions: do not advertise the point-to-point links at all, which works because the EBGP-based design changes the next-hop address at every device so distant networks are reachable via the advertising peer, though this complicates operations and monitoring, since traceroute will display IP addresses that are not reachable; or advertise the point-to-point links but summarise them on every device, which requires an address allocation scheme giving each Tier 1 and Tier 2 device a consecutive block of addresses for its downlinks. Server subnets on Tier 3 devices must be announced into BGP without summarisation on Tier 2 and Tier 1 devices, because summarising subnets in a Clos topology results in route black-holing under a single link failure and must be avoided. Using peer links within the same tier to provide bypass paths is undesirable because of the quadratic complexity of the peering mesh and the waste of ports, and a ring alternative adds extra hops with limited bandwidth and would need special tweaks such as splitting every device into an ASN of its own.
For external connectivity, a dedicated cluster of Tier 3 devices replaced by WAN routers can be used, with the Tier 2 devices in that cluster called Border Routers. They must hide network topology by removing Private Use ASNs from the AS_PATH when advertising paths to WAN routers, which is typically done to avoid ASN collisions between data centres and to give a uniform AS_PATH length for WAN ECMP to anycast prefixes; the process for stripping them is not standardised, but most implementations follow the same logic. They must also originate a default route to the data centre devices, which is the only place a default route can be originated because route summarisation is risky in the unmodified Clos topology, and it is more desirable to readvertise the default route than to originate it through complicated conditional schemes, so that a simultaneous failure of all EBGP sessions to the WAN routers does not black-hole traffic. Route summarisation at the edge is possible if the Border Routers are given a different connectivity model, either a peer mesh of physical links or extra links from Tier 1 devices to at least two Border Routers, both of which make the topology non-uniform.
Every link in the network becomes its own BGP session, and each group of switches gets a number that identifies it. The point of giving every tier its own number is that a route can then be checked for loops the way the Internet checks them, and that a failure in one part of the network stops at that part.
§6ECMP, Convergence and Failure Scope
ECMP is the fundamental load-sharing mechanism of a Clos topology: every lower-tier device uses all of its directly attached upper-tier devices to share traffic destined for the same prefix. The number of ECMP paths between any two Tier 3 devices equals the number of devices in the middle stage, so the BGP implementation must support multipath fan-out up to the maximum number of devices directly attached in the upstream or downstream direction, a number that normally does not exceed half the ports on a device; an ECMP fan-out of 32 would be needed in a Clos built from 64-port devices. If hardware does not support wider ECMP, logical link grouping at Layer 2 can provide hierarchical ECMP, at the cost of a greater risk of flow polarisation since less entropy is available at the second stage. Most implementations declare paths equal if they match up to step (e) of the BGP best path algorithm, and since the proposed design has no underlying IGP, all IGP costs are assumed zero or the same across paths, with policies applied as necessary to equalise vendor-specific attributes such as the MULTI_EXIT_DISC attribute and origin code. Routing loops are unlikely, because BGP best-path selection prefers shorter AS_PATH lengths.
For application load balancing it is desirable to have the same prefix advertised from multiple Tier 3 devices, which produces paths with different AS_PATH values but the same length, so BGP implementations must support load sharing across different neighbouring ASNs when all other attributes are equal. This is sometimes called multipath relax or multipath multiple-AS. Weighted ECMP, which sends more traffic over some paths in the fan-out, may be desirable to compensate for failures or to use paths with more capacity; the prefixes needing it would have to be injected by a remote BGP speaker over a multi-hop session, and weight distribution could be signalled using the technique in the LINK document. Consistent hashing for the ECMP function is often desirable, to minimise the effect of adding or removing a next hop on flow to next-hop affinity, which matters when the device acts as a load balancer; the downside is increased hardware resource utilisation, since more resources such as ternary content-addressable memory space are needed.
On convergence, the document argues that sub-second convergence is achievable if the implementation supports fast EBGP peering session deactivation and timely RIB and FIB updates on link failure. BGP typically relies on an IGP to route around failures inside an AS, but the proposed design has no IGP, so fault detection relies on the BGP keep-alive timeout or link-failure triggers. Relying solely on keep-alives may cause delays on the order of multiple seconds, since the minimum configurable BGP hold timer on many implementations is three seconds, but many implementations can shut down local EBGP sessions in response to the link-down event for the outgoing interface, a feature sometimes called fast fallover, and since links in modern data centres are predominantly point-to-point fibre, a physical interface failure is often detected in milliseconds and triggers reconvergence. Ethernet links may support failure signalling such as Connectivity Fault Management, and some platforms support Bidirectional Forwarding Detection for sub-second failure detection, though both add requirements to vendor software and possibly hardware and may contradict REQ1. The minimum route advertisement interval timer must also be considered, since it spaces out consecutive UPDATE messages; the initial UPDATE messages carrying withdrawn routes are commonly not affected. In a Clos topology each EBGP speaker typically has either one path, because Tier 2 devices do not accept paths from other Tier 2 devices in the same cluster since they share an ASN, or N paths for the same prefix, where N is significantly large, for example 32.
Clos topologies have large fan-outs, which can affect convergence in some cases. When a link between a Tier 3 and a Tier 2 device fails, the Tier 2 device sends UPDATE messages to all upstream Tier 1 devices withdrawing the affected prefixes, and the Tier 1 devices relay these to all downstream Tier 2 devices except the originator. Those Tier 2 devices should wait for all upstream Tier 1 devices to send an UPDATE before removing the prefixes, and if the original or relaying devices introduce delay the result can be UPDATE dispersion lasting multiple seconds. To avoid this, implementations must support update groups, collections of neighbours sharing the same outbound policy, to which the local speaker sends updates synchronously. Because of false positives in route flap dampening implementations, especially under such dispersion events, that feature is not recommended in this design.
Failure impact scope is reduced in two cases. When a link between Tier 2 and Tier 1 devices fails, the Tier 2 device updates its affected ECMP groups and removes the failed link; there is no need to send new information downstream unless the path was selected as best, in which case only an implicit withdraw is needed and forwarding is unaffected. The affected Tier 1 device loses its only path to a particular cluster and withdraws the associated prefixes, which affects only the Tier 2 devices directly connected to it, and the Tier 3 devices are not involved in reconvergence at all. When a Tier 1 device fails, all Tier 2 devices directly attached to it update their ECMP groups for all prefixes from a non-local cluster, and again the Tier 3 devices are not involved. The worst failure impact scope on the control plane is the network as a whole, for instance when a link between Tier 2 and Tier 3 devices fails, but the number of impacted prefixes is much smaller than in an upper-layer failure, and that large scope is a result of the Clos topology rather than of choosing EBGP.
Routing micro-loops can occur. When a downstream device loses all paths for a prefix, it normally has a default route pointing upstream, so a Tier 2 switch can lose a prefix while a Tier 1 switch still points at it, producing a transient loop in which packets bounce between them until the upstream device updates its forwarding tables. To limit the impact, Tier 2 and Tier 1 switches can be configured with static discard or null routes more specific than the default route for prefixes missing during convergence: a summary route covering all server subnets of the underlying Tier 3 devices on Tier 2 switches, and a summary covering the server subnets allocated for the whole data centre on Tier 1 switches. Those discard routes take precedence only for the duration of convergence, until a more specific prefix is learned.
Traffic between servers is spread over all the available paths, and the number of paths is set by how many switches sit in the middle layer. The document then works through what happens when something breaks: how fast it is noticed, how far the change spreads, and how to stop packets bouncing between two switches during the change.
§9Security Considerations
Section 9 states that the design does not introduce any additional security concerns, and points to the general BGP security considerations in RFC 4271 and RFC 4272. Since a data centre is a single-operator domain, the document assumes that edge filtering is in place to prevent attacks against the BGP sessions themselves from outside the perimeter of the data centre. That may be a more feasible option for most deployments than dealing with key management for TCP MD5, or with the lack of implementations of the TCP Authentication Option available at the time of publication. The Generalized TTL Security Mechanism could also be used to further reduce the risk of BGP session spoofing.
The security note is short: the design adds no new exposure, so the ordinary BGP protections apply, and keeping outsiders away from the sessions is easier than managing keys for every one of them.