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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group Peter Psenak 3 Internet Draft Sina Mirtorabi 4 Expiration Date: September 2005 Abhay Roy 5 File name: draft-ietf-ospf-mt-02.txt Liem Nguyen 6 Padma Pillay-Esnault 7 Cisco Systems 9 March 2005 11 Multi-Topology (MT) Routing in OSPF 13 Status of This Memo 15 This document is an Internet-Draft and is subject to all provisions 16 of section 3 of RFC 3667. By submitting this Internet-Draft, each 17 author represents that any applicable patent or other IPR claims of 18 which he or she is aware have been or will be disclosed, and any of 19 which he or she become aware will be disclosed, in accordance with 20 RFC 3668. 22 Internet-Drafts are working documents of the Internet Engineering 23 Task Force (IETF), its areas, and its working groups. Note that 24 other groups may also distribute working documents as 25 Internet-Drafts. 27 Internet-Drafts are draft documents valid for a maximum of six months 28 and may be updated, replaced, or obsoleted by other documents at any 29 time. It is inappropriate to use Internet-Drafts as reference 30 material or to cite them other than as "work in progress." 32 The list of current Internet-Drafts can be accessed at 33 http://www.ietf.org/ietf/1id-abstracts.txt. 35 The list of Internet-Draft Shadow Directories can be accessed at 36 http://www.ietf.org/shadow.html. 38 This Internet-Draft will expire on August 22, 2005. 40 Copyright Notice 42 Copyright (C) The Internet Society (2005). 44 Abstract 46 This draft describes an extension to OSPF in order to define 47 independent IP topologies called Multi-Topologies (MTs). The MT 48 extension can be used for computing different paths for unicast 49 traffic, multicast traffic, different classes of service, or 50 in-band network management. [M-ISIS] describes a similar 51 mechanism for ISIS. An optional extension to exclude 52 selected links from the default topology is also described. 54 1. Introduction 56 OSPF uses a fixed packet format, therefore it is not easy to 57 introduce any backward compatible extensions. However, the OSPF 58 specification [OSPF] introduced TOS metric in an earlier 59 specification [RFC1583] in order to announce a different link cost 60 based on TOS. TOS based routing as described in [RFC1583] was never 61 deployed and was subsequently deprecated. 63 We propose to reuse the TOS based metric fields. They have been 64 redefined as MT-ID and MT-ID Metric and are used to advertise 65 different topologies by advertising separate metrics for each 66 of them. 68 2. Terminology 70 We define the following terminology in this document: 72 Non-MT router : Routers that do not have the MT capability 74 MT router : Routers that have MT capability as described in 75 this document 77 MT-ID : Renamed TOS field in LSAs to represent multi 78 topology ID. 80 Default topology : Topology that is built using the TOS 0 metric 81 (default metric) 83 MT topology : Topology that is built using the corresponding 84 MT-ID metric 86 MT : Shorthand notation for MT topology 88 MT#0 topology : Representation of TOS 0 metric in MT-ID format 90 Non-MT-Area : An area that contains only non-MT routers 92 MT-Area : An area that contains both non-MT routers and MT 93 routers or only MT routers 95 3. MT area boundary 97 Each OSPF interface belongs to a single area and all MTs sharing that 98 link need to belong to the same area. Therefore the area boundaries 99 for all MTs are the same but each MT's attachment to the area is 100 independent. 102 4. Adjacency for MTs 104 Each interface can be configured to belong to a set of topologies. A 105 single adjacency will be formed with neighbors on the interface 106 even if the interface is configured to participate in multiple 107 topologies. Furthermore, adjacency formation will be independent 108 of the topologies configured for the interface or neighbors on that 109 interface. 111 5. Sending OSPF control packets 113 OSPF control packets MUST be sent over the default topology. 115 OSPF control packets sent to the remote end-point of a virtual 116 link may need to traverse multiple hops. These control packets 117 MUST be correctly classified by the virtual link end-point 118 routers as packets belonging to the default topology. Even though 119 the VL may belong to one or more non-default topologies, OSPF control 120 packets sent to the remote end of a virtual link MUST be forwarded 121 using the default topology. 123 6. Advertising MT adjacencies and corresponding IP prefixes 125 We will reuse the TOS metric field in order to advertise a topology 126 and prefixes belonging to that topology. The TOS field is redefined 127 as MT-ID in the payload of Router-LSAs, Summary-LSAs, NSSA-LSAs, 128 and AS-External-LSAs (see Appendix A). 130 MT-ID metrics in LSAs SHOULD be in ascending order of MT-ID. If 131 an MT-ID exists in an LSA or router link multiple times, the metric 132 in the first MT-ID instance MUST be used. 134 6.1 Intra-area routing 136 When a router establishes a FULL adjacency over a link that belongs 137 to a set of MTs, it will advertise the corresponding cost for each 138 MT-ID. 140 By default, all links are included in default topology and all 141 advertised prefixes belonging to the default topology will use 142 the TOS0 metric the same as in standard OSPF [OSPF]. 144 Each MT has its own MT-ID metric field. When a link is not 145 part of a given MT, the corresponding MT-ID metric is excluded from 146 the LSA. 148 The Network-LSA does not contain any MT information since the DR is 149 shared by all MTs. Hence, there is no change to the Network-LSA. 151 6.2 Inter-area and External Routing 153 In Summary-LSAs, NSSA-LSAs, and AS-External-LSAs, the TOS metric 154 fields are defined as MT-ID metric fields and are used in order to 155 advertise prefix and router reachability in the corresponding 156 topology. 158 When a router originates a Summary-LSA, NSSA-LSA, or AS-External-LSA 159 that belongs to a set of MTs, it will include the corresponding cost 160 for each MT-ID. By default, the router participates in the default 161 topology and uses the TOS0 metric for the default topology the same 162 as in standard OSPF [OSPF]. 164 Setting the P-bit in NSSA-LSAs is topology independent and pertains 165 to all MT-ID advertised in the body of the LSA. 167 7. Flushing MT information 169 When a certain link or prefix that existed or was reachable in a 170 certain topology is no longer part of that topology or is unreachable 171 in that topology, a new version of the LSA must be originated 172 excluding metric information representing the link or prefix in that 173 topology. 175 The MT metric in the Router-LSA can also be set to the maximum 176 possible metric to enable the router to become a stub in a certain 177 topology [STUB]. 179 8. MT SPF Computation 181 By considering MT-ID metrics in the LSAs, OSPF will be able to 182 compute multiple topologies and find paths to IP prefixes for each MT 183 independently. A separate SPF will be computed for each MT-ID to find 184 independent paths to IP prefixes. Each nexthop computed during the MT 185 SPF MUST belong to the same MT. 187 Network-LSAs are used by all topologies during the SPF computation. 188 During the SPF for a given MT-ID, only the links and metrics for that 189 MT-ID will be considered. Entries in the Router Routing table will 190 be MT-ID specific. 192 During the SPF computation for the default topology only the TOS0 193 metric is considered during the SPF computation. 195 9. MT ID Values 197 Since AS-External-LSAs use the high order bit in the MT-ID field 198 (E bit) for the external metric-type, only MT-IDs in the range 199 [0-127] are valid. The following MT-ID values are reserved: 201 0 - Reserved for routers in MTRoutingExclusionCapability mode 202 to advertise the metric associated with the default 203 topology (see section 11.2). 205 1 - Reserved for the default multicast topology. 207 Unknown MT-IDs SHOULD be ignored. 209 10. Forwarding in MT 211 Forwarding assures that only routes belonging to a single 212 topology are used to forward a packet along its way from source to 213 destination. Therefore, user configuration MUST be consistently 214 applied throughout the network so that an incoming packet is 215 associated with the same topology through each hop end to end. 216 It is outside of the scope of this document to consider 217 different methods of associating an incoming packet to a 218 corresponding topology. 220 11. Exclusion of links in the default topology 222 The multi-topologies imply that all the routers participate in the 223 default topology. However, it can be useful to exclude some links 224 from the default topology and reserve them for some specific 225 classes of traffic. 227 The multi-topologies extension for default topology link or prefix 228 exclusion is described in the following subsections. 230 11.1 MT-bit in Hello packet 232 OSPF does not have the notion of an unreachable link. All links can 233 have a maximum metric of 0xFFFF advertised in the Router-LSA. The 234 link exclusion capability requires routers to ignore TOS0 metrics in 235 Router-LSAs in the default topology and to alternately use the 236 MT-ID#0 metric to advertise the metric associated with the default 237 topology. Hence, all routers within an area MUST agree on how the 238 metric for default topology will be advertised. 240 The unused T-bit is defined as the MT-bit in the option field 241 in order to assure that a multi-topology link-excluding capable 242 router will only form an adjacency with another similarly configured 243 router. 245 +---+---+---+---+---+---+---+---+ 246 |DN |O |DC |EA |NP |MC |E |MT | 247 +---+---+---+---+---+---+---+---+ 249 MT-bit: This bit MUST be set in the Hello packet only if 250 MTRoutingExclusionCapability is enabled (see section 11.2). 252 11.2 New parameter in the Area Data Structure 254 We define a new parameter in the Area Data Structure: 256 MTRoutingExclusionCapability 257 This is a configurable parameter that will be used to facilitate 258 the introduction of MT routers in an area and ensure backward 259 compatibility. 261 By default, when an area data structure is created the 262 MTRoutingExclusionCapability is disabled. 264 If MTRoutingExclusionCapability is disabled: 265 o The MT-bit MUST be cleared in Hello packets. 266 o If a link participates in a non-default topology, 267 it is automatically included in the default topology 268 to support backward compatibility between MT and 269 non-MT routers. This is accomplished through advertisement 270 via the TOS0 metric field the same as in standard OSPF [OSPF]. 272 If MTRoutingExclusionCapability is enabled: 273 o The MT-bit MUST be set in Hello packets 274 o The router will only accept a Hello if the MT-bit is set (see 275 section 11.3) 277 When MTRoutingExclusionCapability is set to enabled a router is 278 said to be operating in MTRoutingExclusionCapability mode. 280 11.3 Adjacency Formation with Link Exclusion Capability 282 In order to have a smooth transition from a non-MT area to an 283 MT-area, an MT router with MTRoutingExclusionCapability disabled will 284 form adjacencies with non-MT routers and will include all links 285 as part of default topology. 287 A link may cease participating in default topology if 288 MTRoutingExclusionCapability is set to enabled. In this state, a 289 router will only form adjacency with routers that set the MT-bit 290 in their Hello packets. This will ensure that all routers have 291 MTRoutingExclusionCapability enabled before the default topology 292 can be disabled on a link. 294 Receiving OSPF Hello packets as defined in section 10.5 of [OSPF] is 295 modified as follows: 297 o If the MTRoutingExclusionCapability of the Area Data structure 298 is set to enabled, the Hello packets are discarded if the 299 the received Hello packet does not have the MT-bit in the hello 300 options set. 302 11.4 OSPF Control Packets Transmission Over Excluded Links 304 If MTRoutingExclusionCapability is enabled and the default 305 topology is not configured on an interface, connected routes MUST 306 still exist for the default topology and should enable OSPF control 307 packets to be sent and received over that interface. 309 11.5 OSPF LSA Advertisement and SPF Computation for Excluded Links 311 When MTRoutingExclusionCapability is enabled and the link does 312 not participate in the default topology, the MT-ID#0 metric is not 313 advertised. The TOS0 metric is set to infinity (0xFFFF) but is 314 ignored during the default topology SPF computation. 316 When MTRoutingExclusionCapability is enabled and a link participates 317 in the default topology, MT-ID#0 metric is used to advertise the 318 metric associated with the default topology. The TOS0 metric is set 319 to the same value as the MT-ID#0 metric but is ignored during the 320 default topology SPF computation. 322 Independent of the MTRoutingExclusionCapability setting, the TOS0 323 metric is used for Summary-LSAs, NSSA-LSAs, and AS-External-LSAs. 325 o If the prefix or router does not exist in the default topology, 326 the TOS0 metric is set to infinity (0xFFFFFF). 328 o If the prefix or router exists in default the topology, the 329 TOS0 metric is used to advertise the metric in the default 330 topology. 332 During the summary and external prefix calculation for the default 333 topology the TOS0 metric is used for Summary-LSAs, NSSA-LSAs, and 334 AS-External-LSAs. 336 12. Interoperability between MT capable and non-MT capable routers 338 The default metric field is mandatory in all LSAs (even when metric 339 value is 0). Even when a link or prefix does not exist in the 340 default topology, a non-MT router can consider the zero value 341 in the metric field as a valid metric and consider the link or 342 prefix as part of the default topology. 344 In order to prevent the above problem, an MT capable router will 345 include all links as part of the default topology. If links need 346 to be removed from the default topology, an MT capable router 347 MUST be configured in MTRoutingExclusionCapability mode. In this 348 mode, routers will assure that all other routers in the area are 349 in the MTRoutingExclusionCapability mode before considering the 350 MT-ID#0 metric in the SPF calculation. Only then can the TOS0 metric 351 field in Router LSAs be safely ignored during the default topology 352 SPF computation. 354 Note that for any prefix or router to become reachable in a certain 355 topology, a contiguous path inside that topology must exist between 356 the calculating router and the destination prefix or router. 358 13. Migration from non-MT-Area to MT-area 360 Introducing MT-OSPF into a network can be done gradually to allow 361 MT routers and non-MT routers to participate in the default topology 362 while MT routers participate in other topologies. 364 If there is a requirement to exclude some links from the default 365 topology in an area, all routers in the area MUST be in 366 MTRoutingExclusionCapability mode. In this section we describe the 367 migration steps to consider while transitioning from a non-MT network 368 to an MT network. 370 Consider a network with a backbone area and a set of non-backbone 371 areas functioning in standard OSPF mode. We would like to migrate to 372 an MT network either partially or completely. 374 1) As required, part of an area is upgrade to be MT capable. The 375 MT routers will interact with non-MT routers in the default 376 topology and participate in other topologies as required. 378 2) If a new non-backbone area is created for MT routers, it may be 379 configured in MTRoutingExclusionCapability mode since there is no 380 interaction required with non-MT routers. In this mode, the 381 default topology can be excluded on links as required. 383 3) If there is more than one non-backbone areas where MT is being 384 used, it is desirable that the backbone area first be upgraded to 385 be MT capable so that inter-area routing is assured for MT 386 destinations in different areas. 388 4) Gradually the whole network can be made MT capable. 390 Note that inter-area routing for the MT-area still depends on the 391 backbone area. Therefore, if different areas configured for a given 392 topology need to communicate, the backbone area also needs to be 393 configured for this topology. 395 14. Acknowledgments 397 The authors would like to thank Scott Sturgess and Alvaro Retana for 398 their comments on the document. Thanks to Acee Lindem for review and 399 extensive editing. 401 15. Security Consideration 403 The described protocol extension does not introduce any new security 404 issues into the OSPF protocol. 406 16. IANA Considerations 408 The T-bit as defined in [RFC1583] for a router's TOS capability is 409 redefined as the MT-bit in this document. Similarly, the TOS field 410 for Router-LSAs, Summary-LSAs, NSSA-LSAs, and AS-External LSAs as 411 defined in [OSPF] is redefined as MT-ID in this document. 413 17. Normative References 415 [OSPF] Moy, J., "OSPF Version 2", RFC 2328, April 1998. 417 [RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994. 419 [NSSA] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option", 420 RFC 3101, January 2003. 422 [RFC2119] Bradner, S., "Key words for use in RFC's to Indicate 423 Requirement Levels", RFC 2119, March 1997. 425 18. Informative References 427 [M-ISIS] Przygienda, T., Shen, N., Sheth, N., 428 "M-ISIS: Multi Topology (MT) Routing in IS-IS", 429 draft-ietf-isis-wg-multi-topology-07.txt, 430 Work in progress. 432 [STUB] Retana, A., Nguyen, L., White, R., Zinin, A. and D. 433 McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 434 2001. 436 Appendix A. 438 LSA content defined in [OSPF] is modified to introduce the MT-ID. 440 A.1 Router-LSAs 442 Router-LSAs are the Type 1 LSAs. Each router in an area originates 443 a router-LSA. The LSA describes the state and cost of the router's 444 links (i.e., interfaces) to the area. All of the router's links to 445 the area must be described in a single router-LSA. For details 446 concerning the construction of router-LSAs, see Section 447 12.4.1 [OSPF]. 449 0 1 2 3 450 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 1 451 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 452 | LS age | Options | 1 | 453 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 454 | Link State ID | 455 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 456 | Advertising Router | 457 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 458 | LS sequence number | 459 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 460 | LS checksum | length | 461 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 462 |*|*|*|N|W|V|E|B| 0 | # links | 463 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 464 | Link ID | 465 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 466 | Link Data | 467 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 468 | Type | # MT-ID | metric | 469 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 470 | MT-ID | 0 | MT-ID metric | 471 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 472 | ... | 474 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 475 | MT-ID | 0 | MT-ID metric | 476 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 477 | Link ID | 478 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 479 | Link Data | 480 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 481 | ... | 483 A.2 Network-LSAs 485 Network-LSAs are the Type 2 LSAs. A network-LSA is originated for 486 each broadcast and NBMA network in the area which supports two or 487 more routers. The network-LSA is originated by the network's 488 Designated Router. The LSA describes all routers attached to the 489 network, including the Designated Router itself. The LSA's Link 490 State ID field lists the IP interface address of the Designated 491 Router. 493 The distance from the network to all attached routers is zero. This 494 is why metric fields need not be specified in the network-LSA. For 495 details concerning the construction of network-LSAs, see Section 496 12.4.2 [OSPF]. 498 0 1 2 3 499 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 1 500 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 501 | LS age | Options | 2 | 502 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 503 | Link State ID | 504 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 505 | Advertising Router | 506 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 507 | LS sequence number | 508 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 509 | LS checksum | length | 510 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 511 | Network Mask | 512 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 513 | Attached Router | 514 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 515 | ... | 517 Note that network LSA does not contain any MT-ID fields as the cost 518 of the network to the attached routers is 0 and DR is shared by 519 all topologies. 521 A.3 Summary-LSAs 523 Summary-LSAs are the Type 3 and 4 LSAs. These LSAs are originated 524 by area border routers. Summary-LSAs describe inter-area 525 destinations. For details concerning the construction of summary- 526 LSAs, see Section 12.4.3 [OSPF]. 528 Type 3 summary-LSAs are used when the destination is an IP network. 529 In this case the LSA's Link State ID field is an IP network number 530 (if necessary, the Link State ID can also have one or more of the 531 network's "host" bits set; see Appendix E [OSPF] for details). When 532 the destination is an AS boundary router, a Type 4 summary-LSA is 533 used, and the Link State ID field is the AS boundary router's OSPF 534 Router ID. (To see why it is necessary to advertise the location of 535 each ASBR, consult Section 16.4 of [OSPF]). Other than the difference 536 in the Link State ID field, the format of Type 3 and 4 summary-LSAs 537 is identical. 539 0 1 2 3 540 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 1 541 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 542 | LS age | Options | 3 or 4 | 543 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 544 | Link State ID | 545 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 546 | Advertising Router | 547 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 548 | LS sequence number | 549 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 550 | LS checksum | length | 551 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 552 | Network Mask | 553 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 554 | 0 | metric | 555 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 556 | MT-ID | MT-ID metric | 557 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 558 | ... | 559 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 560 | MT-ID | MT-ID metric | 561 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 563 A.4.5 AS-external-LSAs 565 AS-external-LSAs are the Type 5 LSAs. These LSAs are originated by 566 AS boundary routers, and describe destinations external to the AS. 567 For details concerning the construction of AS-external-LSAs, see 568 Section 12.4.3 [OSPF]. 570 AS-external-LSAs usually describe a particular external destination. 571 For these LSAs the Link State ID field specifies an IP network number 572 (if necessary, the Link State ID can also have one or more of the 573 network's "host" bits set; see Appendix E [OSPF] for details). 574 AS-external-LSAs are also used to describe a default route. Default 575 routes are used when no specific route exists to the destination. 576 When describing a default route, the Link State ID is always set to 577 DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0. 579 0 1 2 3 580 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 1 581 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 582 | LS age | Options | 5 | 583 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 584 | Link State ID | 585 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 586 | Advertising Router | 587 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 588 | LS sequence number | 589 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 590 | LS checksum | length | 591 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 592 | Network Mask | 593 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 594 |E| 0 | metric | 595 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 596 | Forwarding address | 597 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 598 | External Route Tag | 599 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 600 |E| MT-ID | MT-ID metric | 601 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 602 | Forwarding address | 603 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 604 | External Route Tag | 605 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 606 | ... | 607 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 608 |E| MT-ID | MT-ID metric | 609 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 610 | Forwarding address | 611 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 612 | External Route Tag | 613 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 615 A.4.6 NSSA-LSAs 617 NSSA-LSAs are the Type 7 LSAs. These LSAs are originated by 618 AS boundary routers local to an NSSA, and describe destinations 619 external to the AS. The changes to NSSA-LSAs are identical to those 620 for External-LSAs (Appendix A.4.5). For details concerning the 621 construction of NSSA-LSAs see Section 2.4 [NSSA]. 623 Authors' address 625 Peter Psenak Abhay Roy 626 Cisco Systems Cisco systems 627 Parc Pegasus, 170 W. Tasman Dr. 628 De Kleetlaan 6A San Jose, CA 95134 629 1831 Diegem, Belgium USA 630 E-mail: ppsenak@cisco.com E-mail: akr@cisco.com 632 Sina Mirtorabi Liem Nguyen 633 Cisco Systems Cisco Systems 634 225 West Tasman drive 7025 Kit Creek Rd. 635 San Jose, CA 95134 Research Triangle Park, NC 27709 636 USA USA 637 E-mail: sina@cisco.com E-mail: lhnguyen@cisco.com 639 Padma Pillay-Esnault 640 Cisco Systems 641 3750 Cisco Way 642 San Jose, CA 95134 643 USA 644 E-mail: ppe@cisco.com 646 Full Copyright Statement 648 Copyright (C) The Internet Society (2004). 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