OSPF-MT: Multi-Topology (MT) Routing in OSPF
Running several independent sets of OSPF paths side by side, one metric and one routing table each.
Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", RFC 4915, DOI 10.17487/RFC4915, June 2007, <https://www.rfc-editor.org/info/rfc4915>.
@misc{rfc4915,
series = {Request for Comments},
number = 4915,
howpublished = {RFC 4915},
publisher = {RFC Editor},
doi = {10.17487/RFC4915},
url = {https://www.rfc-editor.org/info/rfc4915},
author = {P. Psenak and S. Mirtorabi and A. Roy and L. Nguyen and P. Pillay-Esnault},
title = {{Multi-Topology (MT) Routing in OSPF}},
pagetotal = 20,
year = 2007,
month = jun,
} RFC text
Contents
Network Working Group P. Psenak
Request for Comments: 4915 Cisco Systems
Category: Standards Track S. Mirtorabi
Force10 Networks
A. Roy
L. Nguyen
P. Pillay-Esnault
Cisco Systems
June 2007
Multi-Topology (MT) Routing in OSPF
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document describes an extension to Open Shortest Path First
(OSPF) in order to define independent IP topologies called Multi-
Topologies (MTs). The Multi-Topologies extension can be used for
computing different paths for unicast traffic, multicast traffic,
different classes of service based on flexible criteria, or an in-
band network management topology.
An optional extension to exclude selected links from the default
topology is also described.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Differences between Multi-Topology and TOS-Based
Routing . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Notation . . . . . . . . . . . . . . . . . . 3
2.2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Base MT Functional Specifications . . . . . . . . . . . . . . 4
3.1. MT Area Boundary . . . . . . . . . . . . . . . . . . . . . 4
3.2. Adjacency for MTs . . . . . . . . . . . . . . . . . . . . 4
3.3. Sending OSPF Control Packets . . . . . . . . . . . . . . . 5
3.4. Advertising MT Adjacencies and the Corresponding IP
Prefixes . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.4.1. Inter-Area and External Routing . . . . . . . . . . . 5
3.5. Flushing MT Information . . . . . . . . . . . . . . . . . 6
3.6. MT SPF Computation . . . . . . . . . . . . . . . . . . . . 6
3.7. MT-ID Values . . . . . . . . . . . . . . . . . . . . . . . 6
3.8. Forwarding in MT . . . . . . . . . . . . . . . . . . . . . 6
4. Default Topology Link Exclusion Functional Specifications . . 7
4.1. Exclusion of Links in the Default Topology . . . . . . . . 7
4.2. New Area Data Structure Parameter . . . . . . . . . . . . 7
4.3. Adjacency Formation with Link Exclusion Capability . . . . 8
4.4. OSPF Control Packets Transmission over Excluded Links . . 9
4.5. OSPF LSA Advertisement and SPF Computation for
Excluded Links . . . . . . . . . . . . . . . . . . . . . . 9
5. Interoperability between MT-Capable and Non-MT-Capable
Routers . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Demand Circuit Compatibility Considerations . . . . . . . 10
6. Migration from Non-MT-Area to MT-Area . . . . . . . . . . . . 10
7. MT Network Management Considerations . . . . . . . . . . . . . 11
7.1. Create Dedicated Management Topology to Include All
the Nodes . . . . . . . . . . . . . . . . . . . . . . . . 11
7.2. Extend the Default Topology to All the Nodes . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . . 13
Appendix B. OSPF Data Formats . . . . . . . . . . . . . . . . . . 13
B.1. Router-LSAs . . . . . . . . . . . . . . . . . . . . . . . 13
B.2. Network-LSAs . . . . . . . . . . . . . . . . . . . . . . . 15
B.3. Summary-LSAs . . . . . . . . . . . . . . . . . . . . . . . 16
B.4. AS-external-LSAs . . . . . . . . . . . . . . . . . . . . . 17
B.5. Type-7 AS-external-LSAs . . . . . . . . . . . . . . . . . 18
1. Introduction
OSPF uses a fixed packet format, therefore it is not easy to
introduce any backward-compatible extensions. However, the OSPF
specification [OSPF] introduced Type of Service (TOS) metric in an
earlier specification [TOS-OSPF] in order to announce a different
link cost based on TOS. TOS-based routing as described in [TOS-OSPF]
was never deployed and was subsequently deprecated. [M-ISIS]
describes a similar mechanism for ISIS.
We propose to reuse the TOS-based metric fields. They have been
redefined and are used to advertise different topologies by
advertising separate metrics for each of them.
1.1. Differences between Multi-Topology and TOS-Based Routing
Multi-Topology routing differs from [TOS-OSPF] TOS-based routing in
the following ways:
1. With TOS routing [TOS-OSPF], the TOS or Diffserv Code Point
(DSCP) in the IP header is mapped directly to the corresponding
OSPF SPF calculation and routing table. This limits the number
and definition of the topologies to the 16 TOS values specified
in Section 12.3 of [TOS-OSPF]. With Multi-Topology routing, the
classification of what type of traffic maps to which topology is
not within the scope of this document.
2. With TOS routing [TOS-OSPF], traffic that is unreachable in the
routing table associated with the corresponding TOS will revert
to the TOS 0 routing table. With Multi-Topology routing, this is
optional.
3. With TOS routing [TOS-OSPF], individual links or prefixes could
not be excluded from a topology. If the Link State Advertisement
(LSA) options T-bit was set, all links or prefixes were either
advertised explicitly or defaulted to the TOS 0 metric. With
Multi-Topology routing, links or prefixes that are not advertised
for a specific topology do not exist in that topology.
2. Terminology
2.1. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119
[RFC-KEYWORDS].
2.2. Terms
We use the following terminology in this document:
Non-MT router
Routers that do not have the MT capability.
MT router
Routers that have MT capability as described in this document.
MT-ID
Renamed TOS field in LSAs to represent Multi-Topology ID.
Default topology
Topology that is built using the TOS 0 metric (default metric).
MT topology
Topology that is built using the corresponding MT-ID metric.
MT
Shorthand notation for MT topology.
MT#0 topology
Representation of TOS 0 metric in MT-ID format.
Non-MT-Area
An area that contains only non-MT routers.
MT-Area
An area that contains both non-MT routers and MT routers, or only
MT routers.
3. Base MT Functional Specifications
3.1. MT Area Boundary
Each OSPF interface belongs to a single area, and all MTs sharing
that link need to belong to the same area. Therefore, the area
boundaries for all MTs are the same, but each MT's attachment to the
area is independent.
3.2. Adjacency for MTs
Each interface can be configured to belong to a set of topologies. A
single adjacency is formed with neighbors on the interface even if
the interface is configured to participate in multiple topologies.
Furthermore, adjacency formation is independent of the topologies
configured on the local interface and the neighboring router.
3.3. Sending OSPF Control Packets
Sending OSPF control packets is unchanged from [OSPF]. For OSPF
control packets sent to the remote end of a virtual link, the transit
area path MUST be composed of links participating in the default
topology and the OSPF control packets MUST be forwarded using the
default topology.
3.4. Advertising MT Adjacencies and the Corresponding IP Prefixes
The TOS metric field is reused to advertise topology specific metric
for links and prefixes belonging to that topology. The TOS field is
redefined as MT-ID in the payload of Router, Summary, and Type-5 and
Type-7 AS-external-LSAs (see Appendix B).
MT-ID metrics in LSAs SHOULD be in ascending order of MT-ID. If an
MT-ID exists in an LSA or router link multiple times, the metric in
the first MT-ID instance MUST be used.
When a router establishes a FULL adjacency over a link that belongs
to a set of MTs, it advertises the corresponding cost for each MT-ID.
By default, all links are included in the default topology and all
advertised prefixes belonging to the default topology will use the
TOS 0 metric as in [OSPF].
Each MT has its own MT-ID metric field. When a link is not part of a
given MT, the corresponding MT-ID metric is excluded from the LSA.
The Network-LSA does not contain any MT information since the
Designated Router (DR) is shared by all MTs. Hence, there is no
change to the Network-LSA.
3.4.1. Inter-Area and External Routing
In Summary-LSAs and Type-5 and Type-7 AS-external-LSAs, the TOS
metric fields are redefined as MT-ID metric fields and are used to
advertise prefix and router reachability in the corresponding
topology.
When a router originates a Summary-LSA, or Type-5 or Type-7 AS-
external-LSA that belongs to a set of MTs, it includes the
corresponding cost for each MT-ID. By default, the prefix
participates in the default topology and uses the TOS 0 metric for
the default topology, similar to standard OSPF [OSPF].
Setting the P-bit in Type-7 AS-external-LSA is topology independent
and pertains to all MT-ID advertised in the body of the LSA.
3.5. Flushing MT Information
When a certain link or prefix that existed or was reachable in a
certain topology is no longer part of that topology or is unreachable
in that topology, a new version of the LSA MUST be originated
excluding metric information representing the link or prefix in that
topology.
The MT metric in the Router-LSA can also be set to the maximum
possible metric to enable the router to become a stub in a certain
topology [STUB].
3.6. MT SPF Computation
By considering MT-ID metrics in the LSAs, OSPF computes multiple
topologies and finds paths to IP prefixes for each MT independently.
A separate SPF will be computed for each MT-ID to find independent
paths to IP prefixes.
Network-LSAs are used by all topologies during the SPF computation.
During the SPF for a given MT-ID, only the links and metrics for that
MT-ID are considered. Entries in the Router Routing table are also
MT-ID specific.
3.7. MT-ID Values
Since AS-External-LSAs use the high-order bit in the MT-ID field
(E-bit) for the external metric-type, only MT-IDs in the 0 to 127
range are valid. The following MT-ID values are reserved:
0 - Reserved for advertising the metric associated
with the default topology (see Section 4.2)
1 - Reserved for advertising the metric associated
with the default multicast topology
2 - Reserved for IPv4 in-band management purposes
3-31 - Reserved for assignments by IANA
32-127 - Reserved for development, experimental and
proprietary features [RFC3692]
128-255 - Invalid and SHOULD be ignored
3.8. Forwarding in MT
It is outside of the scope of this document to specify how the
information in various topology specific forwarding structures are
used during packet forwarding or how incoming packets are associated
with the corresponding topology. For correct operation, both
forwarding behavior and methods of associating incoming packets to a
corresponding topology must be consistently applied in the network.
4. Default Topology Link Exclusion Functional Specifications
The Multi-Topologies imply that all the routers participate in the
default topology. However, it can be useful to exclude some links
from the default topology and reserve them for some specific classes
of traffic.
The Multi-Topologies extension for the default topology link or
prefix exclusion is described in the following subsections.
4.1. Exclusion of Links in the Default Topology
OSPF does not have the notion of an unreachable link. All links can
have a maximum metric of 0xFFFF advertised in the Router-LSA. The
link exclusion capability requires routers to ignore TOS 0 metrics in
Router-LSAs in the default topology and to alternately use the MT-
ID#0 metric to advertise the metric associated with the default
topology. Hence, all routers within an area MUST agree on how the
metric for the default topology will be advertised.
The unused T-bit is defined as the MT-bit in the option field in
order to ensure that a Multi-Topology link-excluding capable router
will only form an adjacency with another similarly configured router.
+---+---+---+---+---+---+---+---+
|DN |O |DC |EA |NP |MC |E |MT |
+---+---+---+---+---+---+---+---+
Figure 1: OSPF Option Bits
MT-bit: If DefaultExclusionCapability is enabled, the bit MUST
be set in Hello packets and SHOULD be set in Database
Description packet (see Section 4.2).
4.2. New Area Data Structure Parameter
We define a new parameter in the Area Data Structure:
DefaultExclusionCapability
This configurable parameter ensures that all routers in an area
have this capability enabled before the default topology can be
disabled on a router link in the area without causing backward-
compatibility problems.
When an area data structure is created, the
DefaultExclusionCapability is disabled by default.
If DefaultExclusionCapability is disabled:
o The MT-bit MUST be cleared in Hello packets and SHOULD be cleared
in Database Description packets.
o If a link participates in a non-default topology, it is
automatically included in the default topology to support backward
compatibility between MT and non-MT routers. This is accomplished
using the TOS 0 metric field as in [OSPF].
If DefaultExclusionCapability is enabled:
o The MT-bit MUST be set in Hello packets and SHOULD be set in
Database Description packets.
o The router will only accept a Hello packet if the MT-bit is set
(see Section 4.3).
When DefaultExclusionCapability is set to enabled, a router is said
to be operating in DefaultExclusionCapability mode.
4.3. Adjacency Formation with Link Exclusion Capability
In order to have a smooth transition from a non-MT area to an MT-
area, an MT router with DefaultExclusionCapability disabled will form
adjacencies with non-MT routers and will include all links as part of
the default topology.
A link may cease participating in the default topology if
DefaultExclusionCapability is set to enabled. In this state, a
router will only form adjacency with routers that set the MT-bit in
their Hello packets. This will ensure that all routers have
DefaultExclusionCapability enabled before the default topology can be
disabled on a link.
Receiving OSPF Hello packets as defined in Section 10.5 of [OSPF] is
modified as follows:
o If the DefaultExclusionCapability in the Area Data structure is
set to enabled, Hello packets are discarded if the received packet
does not have the MT-bit set in the Header Options.
Receiving OSPF Database Description packets as defined in Section
10.6 of [OSPF] is unchanged. While packet options are validated in
Hello packets, the only option checking performed for Database
Description packets is ensuring that the options do not change during
the database exchange process.
4.4. OSPF Control Packets Transmission over Excluded Links
If DefaultExclusionCapability is enabled, the default topology can be
disabled on an interface. Disabling the default topology on an
interface does not impact the installation of connected routes for
the interface in the default topology. It only affects what a router
advertises in its Router-LSA.
This allows OSPF control packets to be sent and received over an
interface even if the default topology is disabled on the interface.
4.5. OSPF LSA Advertisement and SPF Computation for Excluded Links
When DefaultExclusionCapability is enabled and the link does not
participate in the default topology, the MT-ID#0 metric is not
advertised. The link's TOS 0 metric is ignored during the default
topology SPF computation.
When DefaultExclusionCapability is enabled and a link participates in
the default topology, MT-ID#0 metric is used to advertise the metric
associated with the default topology. The link's TOS 0 metric is
ignored during the default topology SPF computation.
Independent of the DefaultExclusionCapability, the TOS 0 metric is
used for Summary-LSAs and Type-5 and Type-7 AS-external-LSAs.
o If the prefix or router does not exist in the default topology,
the TOS 0 metric is set to infinity (0xFFFFFF).
o If the prefix or router exists in the default topology, the TOS 0
metric is used to advertise the metric in the default topology.
During the summary and external prefix calculation for the default
topology, the TOS 0 metric is used for Summary-LSAs and Type-5 and
Type-7 AS-external-LSAs.
5. Interoperability between MT-Capable and Non-MT-Capable Routers
The default metric field is mandatory in all LSAs (even when the
metric value is 0). Even when a link or prefix does not exist in the
default topology, a non-MT router will consider the zero value in the
metric field as a valid metric and consider the link or prefix as
part of the default topology.
In order to prevent the above problem, an MT-capable router will
include all links as part of the default topology. If links need to
be removed from the default topology, an MT-capable router must be
configured in DefaultExclusionCapability mode. In this mode, routers
will ensure that all other routers in the area are in the
DefaultExclusionCapability mode before considering the MT-ID#0 metric
in the SPF calculation. Only then can the TOS 0 metric field in
Router-LSAs be safely ignored during the default topology SPF
computation.
Note that for any prefix or router to become reachable in a certain
topology, a contiguous path inside that topology must exist between
the calculating router and the destination prefix or router.
5.1. Demand Circuit Compatibility Considerations
A change to an area's DefaultExclusionCapability requires additional
processing for area neighbors that are suppressing Hello packets as
specified in "Extending OSPF to Support Demand Circuits" [DEMAND].
When the DefaultExclusionCapability for an area is changed, Hello
suppression must be disabled for these neighbors for a period of
RouterDeadInterval seconds. This implies that Hello packets are sent
with the DC-bit clear as specified in Section 3.2.1 of [DEMAND]
during this period. After RouterDeadInterval seconds, either the
adjacency will be taken down due to rejection of Hello packets with a
conflicting MT-bit or Hello suppression will be renegotiated.
6. Migration from Non-MT-Area to MT-Area
Introducing MT-OSPF into a network can be done gradually to allow MT
routers and non-MT routers to participate in the default topology
while MT routers participate in other topologies.
If there is a requirement to exclude some links from the default
topology in an area, all routers in the area MUST be in
DefaultExclusionCapability mode. In this section, we describe the
migration steps to consider while transitioning from a non-MT network
to an MT network.
Consider a network with a backbone area and a set of non-backbone
areas functioning in standard OSPF mode. We would like to migrate to
an MT network either partially or completely.
1. As required, part of an area is upgraded to be MT capable. The
MT routers will interact with non-MT routers in the default
topology and participate in other topologies as required.
2. If a new non-backbone area is created for MT routers, it may be
configured in DefaultExclusionCapability mode since there is no
interaction required with non-MT routers. In this mode, the
default topology can be excluded on links as required.
3. If there are several non-backbone areas where MT is being used,
it is desirable that the backbone area first be upgraded to be MT
capable so that inter-area routing is ensured for MT destinations
in different areas.
4. Gradually, the whole network can be made MT capable.
Note that inter-area routing for the MT-area still depends on the
backbone area. Therefore, if different areas configured for a given
topology need to communicate, the backbone area also needs to be
configured for this topology.
7. MT Network Management Considerations
When multiple OSPF topologies exist within a domain, some of the
routers can be configured to participate in a subset of the MTs in
the network. This section discusses some of the options we have to
enable operations or the network management stations to access those
routers.
7.1. Create Dedicated Management Topology to Include All the Nodes
This approach is to set up a dedicated management topology or 'in-
band' management topology. This 'mgmt' topology will include all the
routers need to be managed. The computed routes in the topology will
be installed into the 'mgmt' Routing Information Base (RIB). In the
condition of the 'mgmt' topology uses a set of non-overlapping
address space with the default topology, those 'mgmt' routes can also
be optionally installed into the default RIB. The advantages of
duplicate 'mgmt' routes in both RIBs include: the network management
utilities on the system do not have to be modified to use specific
RIB other than the default RIB; the 'mgmt' topology can share the
same link with the default topology if so designed.
7.2. Extend the Default Topology to All the Nodes
Even in the case in which default topology is not used on some of the
nodes in the IP forwarding, we may want to extend the default
topology to those nodes for the purpose of network management.
Operators SHOULD set a high cost on the links that belong to the
extended portion of the default topology. This way, the IP data
traffic will not be forwarded through those nodes during network
topology changes.
8. Security Considerations
This document does not raise any security issues that are not already
covered in [OSPF].
9. IANA Considerations
The T-bit as defined in [TOS-OSPF] for a router's TOS capability is
redefined as the MT-bit in this document. IANA has assigned the MT-
bit as defined in Section 4.1.
Similarly, the TOS field for Router-LSAs, Summary-LSAs, and Type-5
and Type-7 AS-external-LSAs, as defined in [OSPF], is redefined as
MT-ID in Section 3.7.
IANA created a new registry, "OSPF Multi-Topology ID Values", with
the assignments and registration policies listed in Section 3.7 of
this document.
10. References
10.1. Normative References
[DEMAND] Moy, J., "Extending OSPF to Support Demand Circuits",
RFC 1793, April 1995.
[NSSA] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA)
Option", RFC 3101, January 2003.
[OSPF] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[RFC-KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3692] Narten, T., "Assigning Experimental and Testing
Numbers Considered Useful", RFC 3692, January 2004.
[TOS-OSPF] Moy, J., "OSPF Version 2", RFC 1583, March 1994.
10.2. Informative References
[M-ISIS] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS:
Multi Topology (MT) Routing in IS-IS", Work
in Progress, October 2005.
[STUB] Retana, A., Nguyen, L., White, R., Zinin, A., and D.
McPherson, "OSPF Stub Router Advertisement",
RFC 3137, June 2001.
Appendix A. Acknowledgments
The authors would like to thank Scott Sturgess, Alvaro Retana, David
Kushi, Yakov Rekhter, Tony Przygienda, and Naiming Shen for their
comments on the document. Special thanks to Acee Lindem for editing
and to Tom Henderson for an extensive review during the OSPF Working
Group last call.
Appendix B. OSPF Data Formats
LSA content defined in [OSPF] is modified to introduce the MT-ID.
B.1. Router-LSAs
Router-LSAs are the Type 1 LSAs. Each router in an area originates a
router-LSA. The LSA describes the state and cost of the router's
links (i.e., interfaces) to the area. All of the router's links to
the area must be described in a single router-LSA. For details
concerning the construction of router-LSAs, see Section 12.4.1 of
[OSPF].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|*|*|*|N|W|V|E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # MT-ID | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | 0 | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | 0 | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Figure 2: Router-LSA Format
B.2. Network-LSAs
Network-LSAs are the Type 2 LSAs. A network-LSA is originated for
each broadcast and Non-Broadcast Multi-Access (NBMA) network in the
area that supports two or more routers. The network-LSA is
originated by the network's Designated Router. The LSA describes all
routers attached to the network, including the Designated Router
itself. The LSA's Link State ID field lists the IP interface address
of the Designated Router.
The distance from the network to all attached routers is zero. This
is why metric fields need not be specified in the network-LSA. For
details concerning the construction of network-LSAs, see Section
12.4.2 of [OSPF].
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attached Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Figure 3: Network-LSA Format
Note that network-LSA does not contain any MT-ID fields as the cost
of the network to the attached routers is 0 and DR is shared by all
topologies.
B.3. Summary-LSAs
Summary-LSAs are the Type 3 and 4 LSAs. These LSAs are originated by
area border routers. Summary-LSAs describe inter-area destinations.
For details concerning the construction of summary-LSAs, see Section
12.4.3 of [OSPF].
Type 3 summary-LSAs are used when the destination is an IP network.
In this case the LSA's Link State ID field is an IP network number
(if necessary, the Link State ID can also have one or more of the
network's "host" bits set; see Appendix E of [OSPF] for details).
When the destination is an AS boundary router, a Type 4 summary-LSA
is used, and the Link State ID field is the AS boundary router's OSPF
Router ID. (To see why it is necessary to advertise the location of
each ASBR, consult Section 16.4 of [OSPF].) Other than the
difference in the Link State ID field, the format of Type 3 and 4
summary-LSAs is identical.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 3 or 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Summary-LSA Format
B.4. AS-external-LSAs
AS-external-LSAs are the Type 5 LSAs. These LSAs are originated by
AS boundary routers, and describe destinations external to the AS.
For details concerning the construction of AS-external-LSAs, see
Section 12.4.3 of [OSPF].
AS-external-LSAs usually describe a particular external destination.
For these LSAs, the Link State ID field specifies an IP network
number (if necessary, the Link State ID can also have one or more of
the network's "host" bits set; see Appendix E of [OSPF] for details).
AS-external-LSAs are also used to describe a default route. Default
routes are used when no specific route exists to the destination.
When describing a default route, the Link State ID is always set to
DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS age | Options | 5 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LS checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Network Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| MT-ID | MT-ID metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forwarding address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| External Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: AS-External-LSA Format
B.5. Type-7 AS-external-LSAs
Type-7 AS-external-LSAs are originated by AS boundary routers local
to an NSSA (Not-So-Stubby Area), and describe destinations external
to the AS. The changes to Type-7 AS-external-LSAs are identical to
those for AS-external-LSAs (Appendix A.4.5 of [OSPF]). For details
concerning the construction of Type-7 AS-external-LSAs, see Section
2.4 of [NSSA].
Authors' Addresses
Peter Psenak
Cisco Systems
Mlynske Nivy 43
821 09
Bratislava
Slovakia
EMail: ppsenak@cisco.com
Sina Mirtorabi
Force10 Networks
1440 McCarthy Blvd
Milpitas, CA 95035
USA
EMail: sina@force10networks.com
Abhay Roy
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: akr@cisco.com
Liem Nguyen
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: lhnguyen@cisco.com
Padma Pillay-Esnault
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: ppe@cisco.com
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
ISO Plain Language
On this page
§1Why Reuse the Old TOS Field
OSPF uses a fixed packet format, therefore it is not easy to introduce any backward-compatible extensions. However, the OSPF specification introduced a Type of Service metric in an earlier specification in order to announce a different link cost based on TOS. TOS-based routing as described there was never deployed and was subsequently deprecated. A similar mechanism is described for IS-IS.
The proposal in this document is to reuse the TOS-based metric fields. They have been redefined and are used to advertise different topologies by advertising separate metrics for each of them.
Multi-Topology routing differs from TOS-based routing in three ways. With TOS routing, the TOS or Diffserv Code Point in the IP header was mapped directly to the corresponding OSPF SPF calculation and routing table, which limited the number and definition of the topologies to the 16 TOS values; with Multi-Topology routing, the classification of what type of traffic maps to which topology is not within the scope of the document. With TOS routing, traffic that was unreachable in the routing table associated with the corresponding TOS reverted to the TOS 0 routing table; with Multi-Topology routing this is optional. With TOS routing, individual links or prefixes could not be excluded from a topology: if the Link State Advertisement options T-bit was set, all links or prefixes were either advertised explicitly or defaulted to the TOS 0 metric; with Multi-Topology routing, links or prefixes that are not advertised for a specific topology do not exist in that topology.
OSPF uses a fixed packet format, so it is not easy to add backward-compatible extensions. An earlier specification had a Type of Service metric, so that each type of service could have its own link cost, but that mechanism was never deployed and was later deprecated. This RFC reuses those TOS fields: they are redefined and carry a separate metric for each topology. The differences from the old approach are three. The type of service in the IP header no longer picks the routing table directly, and how traffic is classified is outside the document. Falling back to the default table when a route is missing is now optional. And a link or prefix that is not advertised for a topology simply does not exist in it.
| Aspect | TOS-based routing | Multi-Topology routing |
|---|---|---|
| Mapping to a topology | TOS or DSCP in the IP header picks the routing table | How traffic is classified is out of scope |
| Number of topologies | At most the 16 TOS values | MT-ID values from 0 to 127 |
| Unreachable traffic | Falls back to the TOS 0 table | The fallback is optional |
| Links left out of a topology THIS RFC | Not possible | A link not advertised for a topology does not exist in it |
§2The Terms and the Area Rules
A Non-MT router is a router that does not have the MT capability, and an MT router is one that does. The MT-ID is the renamed TOS field in the link state advertisements, now standing for Multi-Topology ID. The default topology is the topology built using the TOS 0 metric, and an MT topology is one built using the corresponding MT-ID metric. A Non-MT-Area is an area that contains only non-MT routers, and an MT-Area is an area that contains both non-MT routers and MT routers, or only MT routers.
Each OSPF interface belongs to a single area, and all the MTs sharing that link need to belong to the same area. The area boundaries for all MTs are therefore the same, but each MT's attachment to the area is independent. Each interface can be configured to belong to a set of topologies, and a single adjacency is formed with the neighbours on the interface even when the interface takes part in multiple topologies. Adjacency formation is independent of the topologies configured on the local interface and on the neighbouring router.
The RFC names the pieces. A Non-MT router has no multi-topology capability, an MT router does. The MT-ID is the TOS field in the link state advertisements, renamed to stand for a topology number. The default topology is the one built with the TOS 0 metric, and an MT topology is the one built with the metric for its own MT-ID. A Non-MT-Area holds only non-MT routers; an MT-Area holds a mix of MT and non-MT routers, or only MT routers. Each OSPF interface belongs to a single area, and all the topologies sharing that link belong to the same area, so the area boundaries are the same for every topology while each topology attaches to the area independently.
§3Advertising Metrics and Computing Paths
The TOS metric field is reused to advertise the topology specific metric for links and prefixes belonging to that topology. The TOS field is redefined as the MT-ID in the payload of Router, Summary, and Type-5 and Type-7 AS-external link state advertisements. MT-ID metrics in the link state advertisements SHOULD be in ascending order of MT-ID, and if an MT-ID exists in an advertisement or router link multiple times, the metric in the first MT-ID instance MUST be used.
When a router establishes a full adjacency over a link that belongs to a set of topologies, it advertises the corresponding cost for each MT-ID. By default all links are included in the default topology, and all advertised prefixes belonging to the default topology use the TOS 0 metric. Each MT has its own MT-ID metric field, and when a link is not part of a given MT the corresponding MT-ID metric is excluded from the advertisement. The Network-LSA does not contain any MT information, because the designated router is shared by all the MTs, so there is no change to it. In Summary-LSAs and Type-5 and Type-7 AS-external-LSAs, the TOS metric fields are redefined as MT-ID metric fields and are used to advertise prefix and router reachability in the corresponding topology.
When a link or prefix that existed or was reachable in a certain topology is no longer part of that topology, or is unreachable in it, a new version of the link state advertisement MUST be originated, excluding the metric information representing the link or prefix in that topology. The MT metric in the Router-LSA can also be set to the maximum possible metric to enable the router to become a stub in a certain topology.
By considering the MT-ID metrics, OSPF computes multiple topologies and finds paths to IP prefixes for each MT independently: a separate SPF is computed for each MT-ID. Network-LSAs are used by all topologies during the computation, but during the SPF for a given MT-ID only the links and metrics for that MT-ID are considered, and the entries in the router routing table are also MT-ID specific.
Since AS-External-LSAs use the high-order bit in the MT-ID field for the external metric type, only MT-IDs in the range 0 to 127 are valid. The reserved values are 0 for the metric associated with the default topology, 1 for the default multicast topology, 2 for IPv4 in-band management purposes, 3 to 31 for assignments by IANA, 32 to 127 for development, experimental and proprietary features, and 128 to 255 are invalid and SHOULD be ignored.
It is outside the scope of the document to specify how the information in the topology specific forwarding structures is used during packet forwarding, or how incoming packets are associated with the corresponding topology. For correct operation, both the forwarding behaviour and the methods of associating incoming packets with a topology must be applied consistently in the network.
One adjacency is formed with the neighbours on an interface, even when the interface takes part in several topologies. When a router forms a full adjacency over a link, it advertises the cost for each topology in turn, and a link that is not part of a topology leaves that topology's metric out. The Network-LSA is unchanged, because the designated router is shared by all topologies. Each router computes a separate shortest path tree for each topology, using only that topology's links and metrics, and its routing table entries are per topology.
- 0Default topology — Reserved for advertising the metric associated with the default topology.
- 1Default multicast topology — Reserved for the metric associated with the default multicast topology.
- 2IPv4 in-band management — Reserved for IPv4 in-band management purposes.
- 3-31IANA assignments — Reserved for assignments by IANA.
- 32-127Experimental and proprietary — Reserved for development, experimental and proprietary features.
- 128-255Invalid — Invalid and SHOULD be ignored. The high-order bit is used for the external metric type.
§4Holding Links Out of the Default Topology
The Multi-Topologies imply that all the routers participate in the default topology. However, it can be useful to exclude some links from the default topology and reserve them for some specific classes of traffic.
OSPF does not have the notion of an unreachable link: all links can have a maximum metric of 0xFFFF advertised in the Router-LSA. The link exclusion capability therefore requires routers to ignore the TOS 0 metrics in Router-LSAs in the default topology and to alternately use the MT-ID#0 metric to advertise the metric associated with the default topology. All routers within an area MUST agree on how the metric for the default topology will be advertised. The unused T-bit is defined as the MT-bit in the option field, in order to ensure that a Multi-Topology link-excluding capable router will only form an adjacency with another similarly configured router. If the capability is enabled, the MT-bit MUST be set in Hello packets and SHOULD be set in Database Description packets.
A new parameter in the Area Data Structure, DefaultExclusionCapability, ensures that all routers in an area have this capability enabled before the default topology can be disabled on a router link in the area without causing backward-compatibility problems. When an area data structure is created the parameter is disabled by default. While it is disabled, the MT-bit MUST be cleared in Hello packets, and a link that participates in a non-default topology is automatically included in the default topology to support backward compatibility with non-MT routers. When the capability is enabled, the router will only accept a Hello packet if the MT-bit is set.
Disabling the default topology on an interface does not affect the installation of connected routes for the interface in the default topology; it only affects what the router advertises in its Router-LSA. This allows OSPF control packets to be sent and received over an interface even if the default topology is disabled on it.
When the capability is enabled and the link does not participate in the default topology, the MT-ID#0 metric is not advertised and the link's TOS 0 metric is ignored during the default topology SPF computation. When the capability is enabled and the link does participate in the default topology, the MT-ID#0 metric is used to advertise the metric associated with the default topology. Independently of the capability, the TOS 0 metric is used for Summary-LSAs and Type-5 and Type-7 AS-external-LSAs: if the prefix or router does not exist in the default topology the TOS 0 metric is set to infinity, 0xFFFFFF, and if it does exist the TOS 0 metric is used to advertise the metric in the default topology.
Some links may be held out of the default topology and reserved for particular traffic. OSPF has no notion of an unreachable link, so the way to do this is a capability called DefaultExclusionCapability, which makes the routers ignore the TOS 0 metric in Router-LSAs and use the metric for MT-ID 0 instead. A previously unused bit in the OSPF options, the MT-bit, is set in Hello packets so that a router only forms an adjacency with another router that has the same capability. The capability is off by default, and turning it on is what allows a link to leave the default topology. It does not stop OSPF control packets from being sent over that link.
§5Living with Routers That Do Not Know
The default metric field is mandatory in all link state advertisements, even when the metric value is 0. Even when a link or prefix does not exist in the default topology, a non-MT router will consider the zero value in the metric field as a valid metric and consider the link or prefix as part of the default topology.
To prevent that, an MT-capable router will include all links as part of the default topology. If links need to be removed from the default topology, an MT-capable router must be configured in DefaultExclusionCapability mode. In this mode, routers ensure that all other routers in the area are in that mode before considering the MT-ID#0 metric in the SPF calculation. Only then can the TOS 0 metric field in Router-LSAs be safely ignored during the default topology SPF computation.
For any prefix or router to become reachable in a certain topology, a contiguous path inside that topology must exist between the calculating router and the destination prefix or router.
A change to an area's DefaultExclusionCapability requires additional processing for area neighbours that are suppressing Hello packets. When the capability for an area is changed, Hello suppression must be disabled for these neighbours for a period of RouterDeadInterval seconds, with the Hello packets sent with the DC-bit clear during that period. After that time, either the adjacency is taken down because of the rejection of Hello packets with a conflicting MT-bit, or Hello suppression is renegotiated.
Introducing MT-OSPF into a network can be done gradually, so that MT routers and non-MT routers participate in the default topology while the MT routers participate in other topologies. The RFC sets out a migration: part of an area is upgraded to be MT capable, and the MT routers interact with the non-MT routers in the default topology while taking part in other topologies as required. If a new non-backbone area is created for the MT routers, it may be configured in DefaultExclusionCapability mode straight away, since there is no interaction required with non-MT routers, and in that mode the default topology can be excluded on links as required. If there are several non-backbone areas where MT is being used, it is desirable that the backbone area first be upgraded to be MT capable, so that inter-area routing is ensured for MT destinations in different areas. Gradually, the whole network can be made MT capable. Inter-area routing for the MT-area still depends on the backbone area, so if different areas configured for a given topology need to communicate, the backbone area also needs to be configured for that topology.
A non-MT router treats the zero value in the metric field as a valid metric, so it will consider any link or prefix as part of the default topology even when the MT-capable router has left it out. To avoid that, an MT-capable router includes all links in the default topology unless it is configured in DefaultExclusionCapability mode, in which case it waits until every other router in the area is in that mode before it ignores the TOS 0 metric. Migration is gradual: part of an area is upgraded first, a new non-backbone area for the MT routers can be created directly in the capability mode, the backbone area should be upgraded next so that inter-area routing works for the new topologies, and finally the whole network becomes MT capable.
- Upgrade part of an areaThe MT routers interact with the non-MT routers in the default topology and take part in other topologies.
- Create a new area in capability modeA new non-backbone area for the MT routers can use DefaultExclusionCapability from the start.
- Upgrade the backbone nextThe backbone should be MT capable first so that inter-area routing works for MT destinations.
- Make the whole network MT capableGradually every router takes part, and the backbone also needs configuring for each topology in use.
§7Reaching the Routers for Management
When multiple OSPF topologies exist within a domain, some of the routers can be configured to participate in a subset of the topologies in the network. The RFC describes two options for enabling operations staff or network management stations to access those routers.
The first is to set up a dedicated management topology, or in-band management topology, which includes all the routers that need to be managed. The computed routes in that topology are installed into the mgmt Routing Information Base. If the mgmt topology uses a set of non-overlapping address space with the default topology, those mgmt routes can also optionally be installed into the default Routing Information Base. The advantages of duplicating the mgmt routes in both tables are that the network management utilities on the system do not have to be modified to use a table other than the default one, and that the mgmt topology can share the same link with the default topology if it is so designed.
The second is to extend the default topology to all the nodes, even in the case where the default topology is not used for IP forwarding on some of them, for the purpose of network management. Operators SHOULD set a high cost on the links that belong to the extended portion of the default topology, so that IP data traffic is not forwarded through those nodes during network topology changes.
When several topologies exist, some routers may take part in only a subset of them, so operations staff and network management stations need a way to reach those routers. One option is a dedicated management topology that includes every router that has to be managed, with its routes installed in its own routing table, and optionally in the default table as well if the address space does not overlap. The other option is to extend the default topology to the routers that need managing, with a high cost on those links so that ordinary traffic is not sent through them.
§8Security and the Registries
The document does not raise any security issues that are not already covered by the OSPF specification.
For IANA, the T-bit defined in the earlier TOS specification for a router's TOS capability is redefined as the MT-bit in this document, and the TOS field for Router-LSAs, Summary-LSAs, and Type-5 and Type-7 AS-external-LSAs is redefined as the MT-ID. IANA created a new registry, OSPF Multi-Topology ID Values, with the assignments and registration policies listed in the document.
The RFC says it raises no security issues beyond those already covered by OSPF. It also records the IANA work: the previously unused T-bit in the OSPF options is redefined as the MT-bit, the TOS field in the Router, Summary and external link state advertisements is redefined as the MT-ID, and IANA created a registry of OSPF Multi-Topology ID Values.