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rfc:rfc4915

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.

Psenak, et al. Standards Track [Page 1] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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

Psenak, et al. Standards Track [Page 2] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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].

Psenak, et al. Standards Track [Page 3] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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.

Psenak, et al. Standards Track [Page 4] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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.

Psenak, et al. Standards Track [Page 5] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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.

Psenak, et al. Standards Track [Page 6] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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.

Psenak, et al. Standards Track [Page 7] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

 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.

Psenak, et al. Standards Track [Page 8] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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

Psenak, et al. Standards Track [Page 9] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

 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.

Psenak, et al. Standards Track [Page 10] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

 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].

Psenak, et al. Standards Track [Page 11] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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.

Psenak, et al. Standards Track [Page 12] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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].

Psenak, et al. Standards Track [Page 13] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

   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

Psenak, et al. Standards Track [Page 14] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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.

Psenak, et al. Standards Track [Page 15] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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

Psenak, et al. Standards Track [Page 16] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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.

Psenak, et al. Standards Track [Page 17] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

    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].

Psenak, et al. Standards Track [Page 18] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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

Psenak, et al. Standards Track [Page 19] RFC 4915 Multi-Topology (MT) Routing in OSPF June 2007

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 contained in BCP 78, and except as set forth therein, the authors
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Psenak, et al. Standards Track [Page 20]

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