The Complete IS-IS Routing Protocol- P14

The Complete IS-IS Routing Protocol- P14:IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, itswell-structured data formats, its flexibility and easy extensibility are all appealing – IS-ISepitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choicein some of the world’s largest networks. Thus, if one is at all interested in routing, it is wellworth the time and effort to learn IS-IS.
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The Complete IS-IS Routing Protocol- P14380 13. IS-IS Extensions Pennsauken NewYork London New York London Washington Frankfurt Paris IPv6 IPv4FIGURE 13.14. For each network topology a dedicated IS Reach is mesh processed Multi Topology Extension 381 Bytes Type 229 1 Length 1 O A R R 2 Topology-ID O A R R 2 Topology-IDFIGURE 13.15. The Topologies Supported TLV #229 lists the topologies that the link carriesTcpdump outputThe Multi Topology Supported TLV reports that this link can be a member of both the IPv4Unicast (0) and the IPv6 Unicast (2) Topology.02:00:08.223369 Out OSI, IS-IS, length: 82 p2p IIH, hlen: 20, v: 1, pdu-v: 1, sys-id-len: 6 (0), max-area: 3 (0) source-id: 1921.6800.1027, holding time: 27s, circuit-id: 0x01, Flags: [ L1L2 IS ] circuit-id: 0x01, PDU length: 82 Point-to-point Adjacency State TLV #240, length: 15 Adjacency State: Up Extended Local circuit ID: 0x00000001 Neighbor SystemID: 1921.6800.1008 Neighbor Extended Local circuit ID: 0x00000100 Protocols supported TLV #129, length: 2 NLPID(s): IPv4, IPv6 IPv4 Interface address(es) TLV #132, length: 4 IPv4 interface address: 172.16.33.213 IPv6 Interface address(es) TLV #232, length: 16 IPv6 interface address: fe80::2a0:a5ff:fe12:3398 Area address(es) TLV #1, length: 4 Area address (length: 3): 49.0001 Restart Signaling TLV #211, length: 3 Restart Request bit clear, Restart Acknowledgement bit clear Remaining holding time: 0s Multi Topology TLV #229, length: 4 IPv4 unicast Topology (0x000), Flags: [none] IPv6 unicast Topology (0x002), Flags: [none] The IIH reports that it can run IPv4 and IPv6. It lists valid IPv4 and IPv6 addresses andtherefore the router can create valid next-hop entries. So the protocols are listed in theMT Topology TLV #229.382 13. IS-IS Extensions Bytes TLV Type 222 1 TLV Length 1 Reserved 2 Topology-ID Neighbor ID ID Length (6) 1 Metric 3 subTLVs Length 1 optional subTLV Type 1 optional subTLV Length 1 optional subTLV Value 1–240 Neighbor ID ID Length (6) 1 Metric 3 subTLVs Length 1 optional subTLV Type 1 optional subTLV Length 1 optional subTLV Value 1 *FIGURE 13.16. The Multi Topology IS Reachability TLV #222 is similar to the Extended IS Reachabil-ity TLV #22 Each router advertises an adjacency for a common topology adjacency using the MultiTopology IS-Reachability TLV #222 (see Figure 13.16).Tcpdump output02:10:39.192433 OSI, IS-IS, length: 151 L1 LSP, hlen: 27, v: 1, pdu-v: 1, sys-id-len: 6 (0), max-area: 3 (0) lsp-id: 1921.6800.1027.00-00, seq: 0x00000050, lifetime: 1199s chksum: 0x1477 (correct), PDU length: 151, Flags: [ L1L2 IS ] [ ... ] Multi Topology TLV #229, length: 4 IPv4 unicast Topology (0x000), Flags: [none] IPv6 unicast Topology (0x002), Flags: [none] Protocols supported TLV #129, length: 2 Multi Topology Extension 383 NLPID(s): IPv4 (0xcc), IPv6 (0x8e) [ ... ] Multi-Topology IP6 Reachability TLV #237, length: 16 IPv6 unicast Topology (0x002), Flags: [none] IPv6 prefix: 2001:708:0:ff19::/64, Distribution: up, Metric: 250000, Internal Multi Topology IS Reachability TLV #222, length: 13 IPv6 unicast Topology (0x002), Flags: [none] IS Neighbor: 1921.6800.1008.00, Metric: 250000, no sub-TLVs present The tcpdump output also shows that the link IPv6 prefix is not encapsulated in the IP6Reachability TLV #236, but rather in the Multi Topology IP6 Reachability TLV #237.The structure of that TLV is illustrated in Figure 13.17. TLV #237 almost looks identical and also shares the semantics of the IP6 Reach TLV.The only difference is that it gets prepended with the 12-bit Topology ID. A similar clonefor the Extended IPv4 Reachability #135 exists, which is the MT IPv4 Reachability TLV#235, as illustr ...