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'STUB') (Obsoleted by RFC 6987) Summary: 8 errors (**), 0 flaws (~~), 8 warnings (==), 9 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Peter Psenak 2 Internet Draft Sina Mirtorabi 3 Expiration Date: February 2005 Abhay Roy 4 File name: draft-ietf-ospf-mt-01.txt Liem Nguyen 5 Padma Pillay-Esnault 6 Cisco Systems 8 February 2005 10 Multi-Topology (MT) Routing in OSPF 12 Status of This Memo 14 This document is an Internet-Draft and is subject to all provisions 15 of section 3 of RFC 3667. By submitting this Internet-Draft, each 16 author represents that any applicable patent or other IPR claims of 17 which he or she is aware have been or will be disclosed, and any of 18 which he or she become aware will be disclosed, in accordance with 19 RFC 3668. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that 23 other groups may also distribute working documents as 24 Internet-Drafts. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 The list of current Internet-Drafts can be accessed at 32 http://www.ietf.org/ietf/1id-abstracts.txt. 34 The list of Internet-Draft Shadow Directories can be accessed at 35 http://www.ietf.org/shadow.html. 37 This Internet-Draft will expire on August 22, 2005. 39 Copyright Notice 41 Copyright (C) The Internet Society (2005). 43 Abstract 45 This draft describes an extension to OSPF in order to define 46 independent IP topologies called Multi-Topologies (MTs). The MT 47 extension can be used for computing different paths for unicast 48 traffic, multicast traffic, different classes of service, or 49 in-band network management. [M-ISIS] describes a similar 50 mechanism for ISIS. An optional extension to exclude 51 selected links from the default topology is also described. 53 1. Introduction 55 OSPF uses a fixed packet format, therefore it is not easy to 56 introduce any backward compatible extensions. However, the OSPF 57 specification [OSPF] introduced TOS metric in an earlier 58 specification [RFC1583] in order to announce a different link cost 59 based on TOS. TOS based routing as described in [RFC1583] was never 60 deployed and was subsequently deprecated. 62 We propose to reuse the TOS based metric fields. They have been 63 redefined as MT-ID and MT-ID Metric and are used to advertise 64 different topologies by advertising separate metrics for each 65 of them. 67 2. Terminology 69 We define the following terminology in this document: 71 Non-MT router : Routers that do not have the MT capability 73 MT router : Routers that have MT capability as described in 74 this document 76 MT-ID : Renamed TOS field in LSAs to represent multi 77 topology ID. 79 Default topology : Topology that is built using the TOS 0 metric 80 (default metric) 82 MT topology : Topology that is built using the corresponding 83 MT-ID metric 85 MT : Shorthand notation for MT topology 87 MT#0 topology : Representation of TOS 0 metric in MT-ID format 89 Non-MT-Area : An area that contains only non-MT routers 91 MT-Area : An area that contains both non-MT routers and MT 92 routers or only MT routers 94 3. MT area boundary 96 Each OSPF interface belongs to a single area and all MTs sharing that 97 link need to belong to the same area. Therefore the area boundaries 98 for all MTs are the same but each MT's attachment to the area is 99 independent. 101 4. Adjacency for MTs 103 Each interface can be configured to belong to a set of topologies. A 104 single adjacency will be formed with neighbors on the interface 105 even if the interface is configured to participate in multiple 106 topologies. Furthermore, adjacency formation will be independent 107 of the topologies configured for the interface or neighbors on that 108 interface. 110 5. Sending OSPF control packets 112 OSPF control packets MUST be sent over the default topology. 114 OSPF control packets sent to the remote end-point of a virtual 115 link may need to traverse multiple hops. These control packets 116 MUST be correctly classified by the virtual link end-point 117 routers as packets belonging to the default topology. Even though 118 the VL may belong to one or more non-default topologies, OSPF control 119 packets sent to the remote end of a virtual link MUST be forwarded 120 using the default topology. 122 6. Advertising MT adjacencies and corresponding IP prefixes 124 We will reuse the TOS metric field in order to advertise a topology 125 and prefixes belonging to that topology. The TOS field is redefined 126 as MT-ID in the payload of Router-LSAs, Summary-LSAs, NSSA-LSAs, 127 and AS-External-LSAs (see Appendix A). 129 MT-ID metrics in LSAs SHOULD be in ascending order of MT-ID. If 130 an MT-ID exists in an LSA or router link multiple times, the metric 131 in the first MT-ID instance MUST be used. 133 6.1 Intra-area routing 135 When a router establishes a FULL adjacency over a link that belongs 136 to a set of MTs, it will advertise the corresponding cost for each 137 MT-ID. 139 By default, all links are included in default topology and all 140 advertised prefixes belonging to the default topology will use 141 the TOS0 metric the same as in standard OSPF [OSPF]. 143 Each MT has its own MT-ID metric field. When a link is not 144 part of a given MT, the corresponding MT-ID metric is excluded from 145 the LSA. 147 The Network-LSA does not contain any MT information since the DR is 148 shared by all MTs. Hence, there is no change to the Network-LSA. 150 6.2 Inter-area and External Routing 152 In Summary-LSAs, NSSA-LSAs, and AS-External-LSAs, the TOS metric 153 field are defined as MT-ID metric fields and are used in order to 154 advertise prefix and router reachability in the corresponding 155 topology. 157 When a router originates a Summary-LSA, NSSA-LSA, or AS-External-LSA 158 that belongs to a set of MTs, it will include the corresponding cost 159 for each MT-ID. By default, the router participates in the default 160 topology and uses the TOS0 metric for the default topology the same 161 as in standard OSPF [OSPF]. 163 Setting the P-bit in NSSA-LSAs is topology independent and pertains 164 to all MT-ID advertised in the body of the LSA. 166 7. Flushing MT information 168 When a certain link or prefix that existed or was reachable in a 169 certain topology is no longer part of that topology or is unreachable 170 in that topology, a new version of the LSA must be originated 171 excluding metric information representing the link or prefix in that 172 topology. 174 The MT metric in the Router-LSA can also be set to the maximum 175 possible metric to enable the router to become a stub in a certain 176 topology [STUB]. 178 8. MT SPF Computation 180 By considering MT-ID metrics in the LSAs, OSPF will be able to 181 compute multiple topologies and find paths to IP prefixes for each MT 182 independently. A separate SPF will be computed for each MT-ID to find 183 independent paths to IP prefixes. Each nexthop computed during the MT 184 SPF MUST belong to the same MT. 186 Network-LSAs are used by all topologies during the SPF computation. 187 During the SPF for a given MT-ID, only the links and metrics for that 188 MT-ID will be considered. Entries in the Router Routing table will 189 be MT-ID specific. 191 During the SPF computation for the default topology only the TOS0 192 metric is considered during the SPF computation. 194 9. MT ID Values 196 Since AS-External-LSAs use the high order bit in the MT-ID field 197 (E bit) for the external metric-type, only MT-IDs in the range 198 [0-127] are valid. The following MT-ID values are reserved: 200 0 - Reserved for routers in MTRoutingExclusionCapability mode 201 to advertise the metric associated with the default 202 topology (see section 11.2). 204 1 - Reserved for the default multicast topology. 206 Unknown MT-IDs SHOULD be ignored. 208 10. Forwarding in MT 210 Forwarding assures that only routes belonging to a single 211 topology are used to forward a packet along its way from source to 212 destination. Therefore, user configuration MUST be consistently 213 applied throughout the network so that an incoming packet is 214 associated with the same topology through each hop end to end. 215 It is outside of the scope of this document to consider 216 different methods of associating an incoming packet to a 217 corresponding topology. 219 11. Exclusion of links in the default topology 221 The multi-topologies imply that all the routers participate in the 222 default topology. However, it can be useful to exclude some links 223 from the default topology and reserve them for some specific 224 classes of traffic. 226 The multi-topologies extension for default topology link or prefix 227 exclusion is described in the following subsections. 229 11.1 MT-bit in Hello packet 231 OSPF does not have the notion of an unreachable link. All links can 232 have a maximum metric of 0xFFFF advertised in the Router-LSA. The 233 link exclusion capability requires routers to ignore TOS0 metrics in 234 Router-LSAs in the default topology and to alternately use the 235 MT-ID#0 metric to advertise the metric associated with the default 236 topology. Hence, all routers within an area MUST agree on how the 237 metric for default topology will be advertised. 239 The unused T-bit is defined as the MT-bit in the option field 240 in order to assure that a multi-topology link-excluding capable 241 router will only form an adjacency with another similarly configured 242 router. 244 +---+---+---+---+---+---+---+---+ 245 |DN |O |DC |EA |NP |MC |E |MT | 246 +---+---+---+---+---+---+---+---+ 248 MT-bit: This bit MUST be set in the Hello packet only if 249 MTRoutingExclusionCapability is enabled (see section 11.2). 251 11.2 New parameter in the Area Data Structure 253 We define a new parameter in the Area Data Structure: 255 MTRoutingExclusionCapability 256 This is a configurable parameter that will be used to facilitate 257 the introduction of MT routers in an area and ensure backward 258 compatibility. 260 By default, when an area data structure is created the 261 MTRoutingExclusionCapability is disabled. 263 If MTRoutingExclusionCapability is disabled: 264 o The MT-bit MUST be cleared in Hello packets. 265 o If a link participates in a non-default topology, 266 it is automatically included in the default topology 267 to support backward compatibility between MT and 268 non-MT routers. This is accomplished through advertisement 269 via the TOS0 metric field the same as in standard OSPF [OSPF]. 271 If MTRoutingExclusionCapability is enabled: 272 o The MT-bit MUST be set in Hello packets 273 o The router will only accept a Hello if the MT-bit is set (see 274 section 11.3) 276 When MTRoutingExclusionCapability is set to enabled a router is 277 said to be operating in MTRoutingExclusionCapability mode. 279 11.3 Adjacency Formation with Link Exclusion Capability 281 In order to have a smooth transition from a non-MT area to an 282 MT-area, an MT router with MTRoutingExclusionCapability disabled will 283 form adjacencies with non-MT routers and will include all links 284 as part of default topology. 286 A link may cease participating in default topology if 287 MTRoutingExclusionCapability is set to enabled. In this state, a 288 router will only form adjacency with routers that set the MT-bit 289 in their Hello packets. This will ensure that all routers have 290 MTRoutingExclusionCapability enabled before the default topology 291 can be disabled on a link. 293 Receiving OSPF Hello packets as defined in section 10.5 of [OSPF] is 294 modified as follows: 296 o If the MTRoutingExclusionCapability of the Area Data structure 297 is set to enabled, the Hello packets are discarded if the 298 the received Hello packet does not have the MT-bit in the hello 299 options set. 301 11.4 OSPF Control Packets Transmission Over Excluded Links 303 If MTRoutingExclusionCapability is enabled and the default 304 topology is not configured on an interface, connected routes MUST 305 still exist for the default topology and should enable OSPF control 306 packets to be sent and received over that interface. 308 11.5 OSPF LSA Advertisement and SPF Computation for Excluded Links 310 When MTRoutingExclusionCapability is enabled and the link does 311 not participate in the default topology, the MT-ID#0 metric is not 312 advertised. The TOS0 metric is set to infinity (0xFFFF) but is 313 ignored during the default topology SPF computation. 315 When MTRoutingExclusionCapability is enabled and a link participates 316 in the default topology, MT-ID#0 metric is used to advertise the 317 metric associated with the default topology. The TOS0 metric is set 318 to the same value as the MT-ID#0 metric but is ignored during the 319 default topology SPF computation. 321 Independent of the MTRoutingExclusionCapability setting, the TOS0 322 metric is used for Summary-LSAs, NSSA-LSAs, and AS-External-LSAs. 324 o If the prefix or router does not exist in the default topology, 325 the TOS0 metric is set to infinity (0xFFFFFF). 327 o If the prefix or router exists in default the topology, the 328 TOS0 metric is used to advertise the metric in the default 329 topology. 331 During the summary and external prefix calculation for the default 332 topology the TOS0 metric is used for Summary-LSAs, NSSA-LSAs, and 333 AS-External-LSAs. 335 12. Interoperability between MT capable and non-MT capable routers 337 The default metric field is mandatory in all LSAs (even when metric 338 value is 0). Even when a link or prefix does not exist in the 339 default topology, a non-MT router can consider the zero value 340 in the metric field as a valid metric and consider the link or 341 prefix as part of the default topology. 343 In order to prevent the above problem, an MT capable router will 344 include all links as part of the default topology. If links need 345 to be removed from the default topology, an MT capable router 346 MUST be configured in MTRoutingExclusionCapability mode. In this 347 mode, routers will assure that all other routers in the area are 348 in the MTRoutingExclusionCapability mode before considering the 349 MT-ID#0 metric in the SPF calculation. Only then can the TOS0 metric 350 field be safely ignored during the default topology SPF computation. 352 Note that for any prefix or router to become reachable in a certain 353 topology, a contiguous path inside that topology must exist between 354 the calculating router and the destination prefix or router. 356 13. Migration from non-MT-Area to MT-area 358 Introducing MT-OSPF into a network can be done gradually to allow 359 MT routers and non-MT routers to participate in the default topology 360 while MT routers participate in other topologies. 362 If there is a requirement to exclude some links from the default 363 topology in an area, all routers in the area MUST be in 364 MTRoutingExclusionCapability mode. In this section we describe the 365 migration steps to consider while transitioning from a non-MT network 366 to an MT network. 368 Consider a network with a backbone area and a set of non-backbone 369 areas functioning in standard OSPF mode. We would like to migrate to 370 an MT network either partially or completely. 372 1) As required, part of an area is upgrade to be MT capable. The 373 MT routers will interact with non-MT routers in the default 374 topology and participate in other topologies as required. 376 2) If a new non-backbone area is created for MT routers, it may be 377 configured in MTRoutingExclusionCapability mode since there is no 378 interaction required with non-MT routers. In this mode, the 379 default topology can be excluded on links as required. 381 3) If there is more than one non-backbone areas where MT is being 382 used, it is desirable that the backbone area first be upgraded to 383 be MT capable so that inter-area routing is assured for MT 384 destinations in different areas. 386 4) Gradually the whole network can be made MT capable. 388 Note that inter-area routing for the MT-area still depends on the 389 backbone area. Therefore, if different areas configured for a given 390 topology need to communicate, the backbone area also needs to be 391 configured for this topology. 393 14. Acknowledgments 395 The authors would like to thank Scott Sturgess and Alvaro Retana for 396 their comments on the document. Thanks to Acee Lindem for review and 397 extensive editing. 399 15. Security Consideration 401 The described protocol extension does not introduce any new security 402 issues into the OSPF protocol. 404 16. IANA Considerations 406 The T-bit as defined in [RFC1583] for a router's TOS capability is 407 redefined as the MT-bit in this document. Similarly, the TOS field 408 for Router-LSAs, Summary-LSAs, NSSA-LSAs, and AS-External LSAs as 409 defined in [OSPF] is redefined as MT-ID in this document. 411 17. Normative References 413 [M-ISIS] Przygienda, T., Shen, N., Sheth, N., 414 "Multi Topology (MT) Routing in IS-IS", 415 draft-ietf-isis-wg-multi-topology-06.txt, 416 Work in progress. 418 [OSPF] Moy, J., "OSPF Version 2", RFC 2328, April 1998. 420 [RFC1583] Moy, J., "OSPF Version 2", RFC 1583, March 1994. 422 [NSSA] Murphy, P., "The OSPF Not-So-Stubby Area (NSSA) Option", 423 RFC 3101, January 2003. 425 [RFC2119] Bradner, S., "Key words for use in RFC's to Indicate 426 Requirement Levels", RFC 2328, March 1977. 428 18. Informative References 430 [STUB] Retana, A., Nguyen, L., White, R., Zinin, A. and D. 431 McPherson, "OSPF Stub Router Advertisement", RFC 3137, June 432 2001. 434 Appendix A. 436 LSA content defined in [OSPF] is modified to introduce the MT-ID. 438 A.1 Router-LSAs 440 Router-LSAs are the Type 1 LSAs. Each router in an area originates 441 a router-LSA. The LSA describes the state and cost of the router's 442 links (i.e., interfaces) to the area. All of the router's links to 443 the area must be described in a single router-LSA. For details 444 concerning the construction of router-LSAs, see Section 445 12.4.1 [OSPF]. 447 0 1 2 3 448 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 1 449 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 450 | LS age | Options | 1 | 451 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 452 | Link State ID | 453 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 454 | Advertising Router | 455 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 456 | LS sequence number | 457 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 458 | LS checksum | length | 459 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 460 |*|*|*|N|W|V|E|B| 0 | # links | 461 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 462 | Link ID | 463 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 464 | Link Data | 465 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 466 | Type | # MT-ID | metric | 467 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 468 | MT-ID | 0 | MT-ID metric | 469 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 470 | ... | 472 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 473 | MT-ID | 0 | MT-ID metric | 474 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 475 | Link ID | 476 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 477 | Link Data | 478 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 479 | ... | 481 A.2 Network-LSAs 483 Network-LSAs are the Type 2 LSAs. A network-LSA is originated for 484 each broadcast and NBMA network in the area which supports two or 485 more routers. The network-LSA is originated by the network's 486 Designated Router. The LSA describes all routers attached to the 487 network, including the Designated Router itself. The LSA's Link 488 State ID field lists the IP interface address of the Designated 489 Router. 491 The distance from the network to all attached routers is zero. This 492 is why metric fields need not be specified in the network-LSA. For 493 details concerning the construction of network-LSAs, see Section 494 12.4.2 [OSPF]. 496 0 1 2 3 497 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 1 498 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 499 | LS age | Options | 2 | 500 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 501 | Link State ID | 502 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 503 | Advertising Router | 504 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 505 | LS sequence number | 506 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 507 | LS checksum | length | 508 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 509 | Network Mask | 510 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 511 | Attached Router | 512 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 513 | ... | 515 Note that network LSA does not contain any MT-ID fields as the cost 516 of the network to the attached routers is 0 and DR is shared by 517 all topologies. 519 A.3 Summary-LSAs 521 Summary-LSAs are the Type 3 and 4 LSAs. These LSAs are originated 522 by area border routers. Summary-LSAs describe inter-area 523 destinations. For details concerning the construction of summary- 524 LSAs, see Section 12.4.3 [OSPF]. 526 Type 3 summary-LSAs are used when the destination is an IP network. 527 In this case the LSA's Link State ID field is an IP network number 528 (if necessary, the Link State ID can also have one or more of the 529 network's "host" bits set; see Appendix E [OSPF] for details). When 530 the destination is an AS boundary router, a Type 4 summary-LSA is 531 used, and the Link State ID field is the AS boundary router's OSPF 532 Router ID. (To see why it is necessary to advertise the location of 533 each ASBR, consult Section 16.4 of [OSPF]). Other than the difference 534 in the Link State ID field, the format of Type 3 and 4 summary-LSAs 535 is identical. 537 0 1 2 3 538 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 1 539 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 540 | LS age | Options | 3 or 4 | 541 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 542 | Link State ID | 543 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 544 | Advertising Router | 545 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 546 | LS sequence number | 547 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 548 | LS checksum | length | 549 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 550 | Network Mask | 551 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 552 | 0 | metric | 553 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 554 | MT-ID | MT-ID metric | 555 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 556 | ... | 557 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 558 | MT-ID | MT-ID metric | 559 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 561 A.4.5 AS-external-LSAs 563 AS-external-LSAs are the Type 5 LSAs. These LSAs are originated by 564 AS boundary routers, and describe destinations external to the AS. 565 For details concerning the construction of AS-external-LSAs, see 566 Section 12.4.3 [OSPF]. 568 AS-external-LSAs usually describe a particular external destination. 569 For these LSAs the Link State ID field specifies an IP network number 570 (if necessary, the Link State ID can also have one or more of the 571 network's "host" bits set; see Appendix E [OSPF] for details). 572 AS-external-LSAs are also used to describe a default route. Default 573 routes are used when no specific route exists to the destination. 574 When describing a default route, the Link State ID is always set to 575 DefaultDestination (0.0.0.0) and the Network Mask is set to 0.0.0.0. 577 0 1 2 3 578 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 1 579 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 580 | LS age | Options | 5 | 581 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 582 | Link State ID | 583 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 584 | Advertising Router | 585 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 586 | LS sequence number | 587 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 588 | LS checksum | length | 589 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 590 | Network Mask | 591 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 592 |E| 0 | metric | 593 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 594 | Forwarding address | 595 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 596 | External Route Tag | 597 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 598 |E| MT-ID | MT-ID metric | 599 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 600 | Forwarding address | 601 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 602 | External Route Tag | 603 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 604 | ... | 605 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 606 |E| MT-ID | MT-ID metric | 607 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 608 | Forwarding address | 609 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 610 | External Route Tag | 611 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 613 A.4.6 NSSA-LSAs 615 NSSA-LSAs are the Type 7 LSAs. These LSAs are originated by 616 AS boundary routers local to an NSSA, and describe destinations 617 external to the AS. The changes to NSSA-LSAs are identical to those 618 for External-LSAs (Appendix A.4.5). For details concerning the 619 construction of NSSA-LSAs see Section 2.4 [NSSA]. 621 Authors' address 623 Peter Psenak Abhay Roy 624 Cisco Systems Cisco systems 625 Parc Pegasus, 170 W. Tasman Dr. 626 De Kleetlaan 6A San Jose, CA 95134 627 1831 Diegem, Belgium USA 628 E-mail: ppsenak@cisco.com E-mail: akr@cisco.com 630 Sina Mirtorabi Liem Nguyen 631 Cisco Systems Cisco Systems 632 225 West Tasman drive 7025 Kit Creek Rd. 633 San Jose, CA 95134 Research Triangle Park, NC 27709 634 USA USA 635 E-mail: sina@cisco.com E-mail: lhnguyen@cisco.com 637 Padma Pillay-Esnault 638 Cisco Systems 639 3750 Cisco Way 640 San Jose, CA 95134 641 USA 642 E-mail: ppe@cisco.com 644 Full Copyright Statement 646 Copyright (C) The Internet Society (2004). This document is subject 647 to the rights, licenses and restrictions contained in BCP 78, and 648 except as set forth therein, the authors retain all their rights. 650 This document and the information contained herein are provided on an 651 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 652 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET 653 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, 654 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE 655 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 656 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 658 Intellectual Property 660 The IETF takes no position regarding the validity or scope of any 661 Intellectual Property Rights or other rights that might be claimed to 662 pertain to the implementation or use of the technology described in 663 this document or the extent to which any license under such rights 664 might or might not be available; nor does it represent that it has 665 made any independent effort to identify any such rights. 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