Network Working Group Peter Psenak Internet Draft Sina Mirtorabi Expiration Date:
AprilFebruary 2005 Abhay Roy File name: draft-ietf-ospf-mt-00.txtdraft-ietf-ospf-mt-01.txt Liem Nguyen Cisco SystemsPadma Pillay-Esnault Juniper Networks October 2004 MT-OSPF: Multi TopologyCisco Systems February 2005 Multi-Topology (MT) Routing in OSPF Status of This Memo This document is an Internet-Draft and is in full conformance withsubject to all provisions of Section 10section 3 of RFC2026. Internet DraftsRFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas,areas, and its Working Groups.working groups. Note that other groups may also distribute working documents as Internet Drafts. Internet DraftsInternet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months. Internet Draftsmonths and may be updated, replaced, or obsoleted by other documents at any time. It is not appropriateinappropriate to use Internet DraftsInternet-Drafts as reference material or to cite them other than as a "working draft" or"work in progress".progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txthttp://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 22, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This draft describes thean extension to OSPF in order to define independent IP topologies called Multi-Topologies (MTs). The MT extension can be used for computing different paths for unicast traffic, multicast traffic, different classes of service, or in-band managementnetwork or incongruent topologies for unicast and multicast. M-ISISmanagement. [M-ISIS] describes a similar mechanism for ISIS. This draft also describes anAn optional extension of Multi-topologies whereby someto exclude selected links might be excludedfrom the default topology.topology is also described. 1. Introduction OSPF uses a fixed packet format, therefore it is not easy to introduce any backward compatible extensions. HoweverHowever, the OSPF specification [OSPF] introduced TOS metric in an earlier specification [RFC1583] in order to announce a different link'slink cost based on TOS. TheTOS based routing as described in [RFC1583] was never deployed in the fieldand laterwas removed from the spec.subsequently deprecated. We propose to reuse the TOS based metric fields. They have been redefined as MT-ID and MT-ID Metric,Metric and are used to announceadvertise different topologies by advertising separate metrics for each of them. 2. Terminology We define 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. 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. 4. Adjacency for MTs Each interface can be configured to belong to a set of topologies. A single adjacency will be formed with neighbors on the remote neighborinterface even if the interface is configured to participate in multiple topologies and independentlytopologies. Furthermore, adjacency formation will be independent of the MT-IDs.topologies configured for the interface or neighbors on that interface. 5. Sending OSPF control packets OSPF control packets shouldMUST be sent over the default topology. OSPF control packets sent to the remote end-point of thea virtual link may need to traverse multiple hops. These control packets shouldMUST be correctly classified by the virtual link end-point routers as packets belonging to the default topology. EventEven though the VL may belong to other than default topology (or multiple of them),one or more non-default topologies, OSPF control packets sent to the remote end of thea virtual link shouldMUST be forwarded using the default topology. 6. Advertising MT adjacencies and corresponding IP prefixes We will reuse the TOS metric field in order to announceadvertise a topology orand prefixes that belongsbelonging to a given MT.that topology. The TOS field is renamed toredefined as MT-ID in the LSAspayload of Router-LSAs, Summary-LSAs, NSSA-LSAs, and AS-External-LSAs (see Appendix A). 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. 6.1 Intra-area routing When a router establishes a FULL adjacency over a link that belongs to a set of MTs, it will advertise the corresponding cost for each MT-ID. AllBy default, all links are by defaultincluded in default topology,topology and all advertised adjacencyprefixes belonging to the default topology will use the TOS0 metric the same as in standard OSPF.OSPF [OSPF]. Each MT has its own MT-ID metric field and whenfield. When a link is not part of a given MT, the corresponding MT-ID metric will not appear inis excluded from the LSA. The Network LSANetwork-LSA does not contain any MT information assince the DR is shared by all MTs and thusMTs. Hence, there is no change to the Network LSA.Network-LSA. 6.2 Inter-area and external routingExternal Routing In SummarySummary-LSAs, NSSA-LSAs, and External LSAs,AS-External-LSAs, the TOS metric fieldsfield are renamed todefined as MT-ID metric fields and are used in order to announce prefix/routeradvertise prefix and router reachability in the corresponding topology. When a router originates a type 3/4/5/7 LSASummary-LSA, NSSA-LSA, or AS-External-LSA that belongs to a set of MTs, it will include the corresponding cost for each MT-ID. TheBy default, the router by default participateparticipates in the default topology and useuses the TOS0 metric for the default topology the same as in standard OSPF.OSPF [OSPF]. Setting the P-bit in NSSA-LSAs is topology independent and pertains to all MT-ID advertised in the body of the LSA. 7. Flushing MT information When a certain link/prefixlink or prefix that existed or was reachable in a certain topology is no longer part of thisthat topology or the reachability of the link/prefixis unreachable in this topology was lost,that topology, a new version of the LSA that advertised the link/prefixmust be originated. This new version of the LSA must not include anyoriginated excluding metric information representing the link/prefixlink or prefix in thisthat 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]. 8. MT SPF Computation By considering MT-ID metrics in the LSAs, OSPF will be able to compute multiple topologies, one for each MT the router is part oftopologies and find 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. Each nexthop computed during the MT SPF MUST belong to the same MT. Network LSAsNetwork-LSAs are used by all topologies during the SPF computation. During the SPF for a given MT-ID, only the link/metriclinks and metrics for the giventhat MT-ID will be considered. Entries in the Router Routing table will be MT-ID specific. During the SPF computation for the default topology only the TOS0 metric is considered during the SPF computation. 9. MT ID Values Only MT-IDs in the range [0-127] are valid, because external LSAsSince AS-External-LSAs use onethe high order bit in the MT-ID field (E bit) for the external metric-type. Followingmetric-type, only MT-IDs in the range [0-127] are valid. The following MT-ID values are reserved: 0 - reservedReserved for routers in MTRoutingExclusionCapability mode to advertise the metric associated with the default topology (see section 11.2). 1 - reservedReserved for the default multicast topology. Any unknown MT-ID shouldUnknown MT-IDs SHOULD be ignored. 10. Forwarding in MT Forwarding must make sureassures that only routes belonging to thea single topology are used to forward thea packet along its way from source to destination, thereforedestination. Therefore, user configuration MUST be consistently applied throughout the network so that an incoming packet is associated with the same topology onthrough each hop as it is being forwarded.end to end. It is outside of the scope of this document to consider different methods of associating an incoming packet to thea corresponding MT.topology. 11. Exclusion of links in the default topology The multi-topologies imply that all the routers participate in the default topology. However, it iscan be useful in some circumstancesto exclude some links from the default topology and reserve them for some specific classes of traffic. The multi-topologies extension for default topology link or prefix exclusion is described in the following sections.subsections. 11.1 MT-bit in Hello packet OSPF does not have the notion of an unreachable link. All links can have a maximum metric of 0xFFFF carriedadvertised in the Router LSA.Router-LSA. The link exclusion capability requires routers to ignore TOS 0 metricTOS0 metrics in router-LSARouter-LSAs in the default topology and to alternately use the MT-ID#0 metric insteadto advertise the metric associated with the default topology. Hence, all routers within an area MUST agree on how the metric for default topology will be advertised. The unused T-bit is renamed (MT)defined as the MT-bit in the option field in order to enforceassure that a multi-topology link-excluding capable router will only interactform an adjacency with another similarly configured router. +---+---+---+---+---+---+---+---+ |DN |O |DC |EA |NP |MC |E |MT | +---+---+---+---+---+---+---+---+ MT-bit: This bit MUST be set in the Hello packet only if MTRoutingExclusionCapability is Enabledenabled (see section 11.2). 11.2 New parameter in the Area Data Structure We define a new parametersparameter in the Area Data Structure: MTRoutingExclusionCapability This is a configurable parameter that will be used to facilitate the introduction of MT routers in an area and ensure thebackward compatibility. By default, when an area data structure is created the MTRoutingExclusionCapabiltyMTRoutingExclusionCapability is disabled. If MTRoutingExclusionCapability is disabled: o The MT-bit MUST be cleared in theHello packetpackets. o If a link participates in a non-default topology, it is automatically included in the default topology (by usingto support backward compatibility between MT and non-MT routers. This is accomplished through advertisement via the defaultTOS0 metric field the same as it is donein standard OSPF ) so that MT routers interact correctly with non-MT routers.[OSPF]. If MTRoutingExclusionCapability is set to Enabled:enabled: o The MT-bit MUST be set in theHello packetpackets o The router will only accept a Hello if the MT-bit is set (see section 11.3) We call MTRoutingExclusionCapability "mode", whenWhen MTRoutingExclusionCapability is set to Enabled. 11.3. Forming adjacencyenabled a router is said to be operating in MTRoutingExclusionCapability mode. 11.3 Adjacency Formation with link exclusion capability.Link Exclusion Capability In order to have a smooth transition from a non-MT area to an MT-area, aan MT router with MTRoutingExclusionCapability set to disabledisabled will form adjacencyadjacencies with non-MT routers and itwill include all links as part of default topology. A link canmay cease participating in default topology if MTRoutingExclusionCapability is set to Enabled.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 are in Enabled modehave MTRoutingExclusionCapability enabled before the default topology can be disabled on a link. Receiving OSPF Hello packets as defined in section 8.210.5 of  are[OSPF] is modified as follows: o If the MTRoutingExclusionCapability of the Area Data structure is set to Enabled,enabled, the Hello packets are discarded if: oif the the received Hello packet does not have the MT-bit set 11.4. Sendingin the hello options set. 11.4 OSPF control packets over an excluded link.Control Packets Transmission Over Excluded Links If MTRoutingExclusionCapability is set to Enabledenabled and the default topology is not configured on thean interface, connected route shouldroutes MUST still exist for athe default topology that wouldand should enable theOSPF control packets to be sent and received. 11.5. Modified MTreceived over that interface. 11.5 OSPF LSA Advertisement and SPF Computation with link exclusion capability.for Excluded Links When MTRoutingExclusionCapability is set to Enabled, MT#0 can be removed if aenabled and the link does not participate in the default topology. In that casetopology, the MT-ID#0 metric is not advertised. The TOS0 metric is set to infinity (0xFFFF) andbut is ignored during the MT#0default topology SPF computation. When MTRoutingExclusionCapability is set to Enabledenabled and a link participates in the default topology, MT-ID#0 metric is used to advertise the metric associated with the default topology. FurtherThe TOS0 metric is set to the same value as the MT-ID#0 metric. However TOS 0metric but is ignored during SPF forthe default topology and only MT-ID#0SPF computation. Independent of the MTRoutingExclusionCapability setting, the TOS0 metric is used for SPF in default topology. When originating SummarySummary-LSAs, NSSA-LSAs, and External LSAs, if MTRoutingExclusionCapability is set to Enabled:AS-External-LSAs. o ifIf the prefix /or router does not exist in the default topology, the TOS0 metric is set to infinity (0xFFFFFF). o ifIf the prefix /or router existexists in default the topology, the TOS0 metric is used to announce a prefix / routeradvertise the metric in the default topology. During the Summarysummary and Externalexternal prefix calculation for the default topology the TOS0 metric is used in LSA Type-3/4/5/7.for Summary-LSAs, NSSA-LSAs, and AS-External-LSAs. 12. Interoperability between MT capable and non-MT capable routers The default metric field is mandatory in all LSAs (even when metric value is 0). Even when thea link or aprefix does not exist in the default topology, a non MT capablenon-MT router can consider the zero value in the metric field as a valid metric and consider the link/prefixlink or prefix as part of the default topology. In order to prevent the above problem, aan MT capable router will by defaultinclude all links as part of the default topology. If links need to be removed from the default topology, aan MT capable router MUST be configured in MTRoutingExclusionCapability mode. In this mode the routermode, routers will make sureassure that all other routers in the area are in the MTRoutingExclusionCapability mode before forming any adjacency so thatconsidering the MT-ID#0 metric in the SPF calculation. Only then can the TOS0 metric field canbe safely ignored during the MT#0default 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. 13. Migration from non-MT-Area to MT-area Introducing MT-OSPF ininto a network can be graduallydone sincegradually to allow MT routers will interactand non-MT routers to participate in the default topology with non-MT routers, yet exchanging information about other topologies with otherwhile MT capable routers.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 MTRoutingExclusionCapability mode. In this section we describe migrationsthe migration steps to consider while transitioning from a non-MT network to aan MT network. Migration Steps ---------------Consider a network with a backbone area and a setsset of non-backbone areas functioning in standard OSPF mode. We would like to migrate to aan MT network either partially or completely. 1) PartAs required, part of an area is upgraded as neededupgrade to havebe MT capability, thecapable. The MT routers will interact with non-MT routers in the default topology, further MT routers willtopology and participate in MT topologyother topologies as needed.required. 2) If a new non-backbone area is created for MT routers, it may be setconfigured in MTRoutingExclusionCapability mode assince there is no interaction required with non-MT routers, inrouters. In this modemode, the default topology can be excluded ifon links as required. 3) If there is more than one non-backbone areas where MT is being used, it is desirable that the backbone area 0 befirst be upgraded to be MT capable routersso that inter-area routing is assured for MT destinations in different areas. 4) Gradually the whole network can be made MT awarecapable. Note that Inter-areainter-area routing for the MT-area still depends on the backbone area. ThereforeTherefore, if different areas inconfigured for a given MT-IDtopology need to communicate, the backbone area also needs to be configured for this MT-ID.topology. 14. Acknowledgments The authors would like to thank Scott Sturgess and Alvaro Retana for their comments on the document. Thanks to Acee Lindem for review and extensive editing. 15. Security Consideration No specificThe described protocol extension does not introduce any new security issues withinto the proposed solutions are known.OSPF protocol. 16. IANA Considerations The T-bit as defined in [RFC1583] for a router's TOS capability is reclaimedredefined as the MT-bit in this document. Likewise,Similarly, the TOS field in type 1,3,4,5,7for Router-LSAs, Summary-LSAs, NSSA-LSAs, and AS-External LSAs as defined in [OSPF] is reclaimedredefined as MT-ID in this document. 17. Normative References [M-ISIS] Przygienda, T., Shen, N., Sheth, N., "Multi Topology (MT) Routing in IS-IS", draft-ietf-isis-wg-multi-topology-06.txt, Work in progress. [OSPF] Moy, J., "OSPF Version 2", RFC 2328, April 1998. [RFC1583] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc.,March 1994. [NSSA] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option", RFC 3101, January 2003. [RFC2119] Bradner, S., "Key words for use in RFC's to Indicate Requirement Levels", RFC 2328, March 1977. 18. Informative References [STUB] Retana, A., Nguyen, L., White, R., Zinin, A. and D. McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 2001. Appendix A. LSAsLSA content defined in  are[OSPF] is modified to introduce the MT-ID. A.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. 12.4.1 [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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | A.2 Network-LSAs Network-LSAs are the Type 2 LSAs. A network-LSA is originated for each broadcast and NBMA network in the area which 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. 12.4.2 [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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | Note that network LSA does not contain any MT-ID fieldfields as the cost of the network to the attached routers is 0 and DR is shared by all MT.topologies. A.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. 12.4.3 [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 [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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A.4.5 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. 12.4.3 [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 [OSPF] for details). AS- external-LSAsAS-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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ A.4.6 NSSA-LSAs NSSA-LSAs are the Type 7 LSAs. These LSAs are originated by AS boundary routers local to an NSSA, and describe destinations external to the AS. The changes to NSSA-LSAs are identical to those for External-LSAs (Appendix A.4.5). For details concerning the construction of NSSA-LSAs see Section 2.4 [NSSA]. Authors' address Peter Psenak Abhay Roy Cisco Systems Cisco systems Parc Pegasus, 170 W. Tasman Dr. De Kleetlaan 6A San Jose, CA 95134 1831 Diegem, Belgium USA E-mail: email@example.com E-mail: firstname.lastname@example.org Sina Mirtorabi Liem Nguyen Cisco Systems Cisco Systems 225 West Tasman drive 7025 Kit Creek Rd. San Jose, CA 95134 Research Triangle Park, NC 27709 USA USA E-mail: email@example.com E-mail: firstname.lastname@example.org Padma PIllay-Esnault Juniper Networks 1194 N. Mathilda Avenue Sunnyvale,Pillay-Esnault Cisco Systems 3750 Cisco Way San Jose, CA 9408995134 USA E-mail: email@example.com@cisco.com Full Copyright Statement Copyright (C) The Internet Society (2004). 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