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1、The IS-IS Protocol,Introducing IS-IS and Integrated IS-IS Routing,Uses for IS-IS Routing,Large ISPsStable protocolOriginally deployed by ISPs because U.S. government mandated Internet support of OSI and IP,IS-IS Routing,IS = router.IS-IS was originally designed as the IGP for the Connectionless Netw
2、ork Service (CLNS), part of the OSI protocol suite.The OSI protocol suite layer 3 protocol is the Connectionless Network Protocol (CLNP).IS-IS uses CLNS addresses to identify routers and build the LSDB.,IS-IS Features,Link-state routing protocolSupports VLSMUses Dijkstras SPF algorithm; has fast con
3、vergenceUses hellos to establish adjacencies and LSPs to exchange link-state informationEfficient use of bandwidth, memory, and processorSupports two routing levels:Level 1: Builds common topology of system IDs in local area and routes within area using lowest cost path. Level 2: Exchanges prefix in
4、formation (area addresses) between areas. Routes traffic to area using lowest-cost path.,Routers are identified as Level 1, Level 2, or Level 1-2:Level 1 routers use LSPs to build topology for local area.Level 2 routers use LSPs to build topology betweendifferent areas.Level 1-2 routers act as borde
5、r routers between Level 1 and Level 2 routing domains.,IS-IS Link-State Operation,Integrated (or Dual) IS-IS Routing,Integrated IS-IS is IS-IS for multiple protocols:For IP, CLNS, or bothUses its own PDUs to transport IP routing information; updates not sent in IP packetsRequires CLNS addresses, eve
6、n if only routing for IP,IP and CLNP addressesmust be planned.Use two-level hierarchy for scalability:Limits LSP floodingProvides opportunity for summarization,Summarization:Limits update trafficMinimizes routermemory and CPU usage,Integrated IS-IS Design Principles,Issues with Integrated IS-IS,Defa
7、ult narrow metrics are limited to 6-bit interface and 10-bit path metricIn Cisco IOS Software Relase 12.0, wide metrics allow 24-bit interface and 32-bit path metric.Cisco IOS software has default metric of 10 on all interfaces.,End System-to-Intermediate System,ES-IS forms adjacencies between ESs a
8、nd routers (ISs).IP end-systems do not use ES-IS.ESs transmit ESHs to announce their presence to ISs.ISs transmit ISHs to announce their presence to ESs.ISs transmit IIHs to other ISs.,Four OSI Routing Levels,Similarities Between IS-IS and OSPF,Integrated IS-IS and OSPF are both open standard link-s
9、tate protocols with the following similar features:Link-state representation, aging timers, and LSDB synchronizationSPF algorithmsUpdate, decision, and flooding processes VLSM supportScalability of link-state protocols has been proven (used in ISP backbones).They both converge quickly after changes.
10、,Integrated IS-IS vs. OSPF: Area Design,OSPF is based on a central backbone with all other areas attached to it. In OSPF the border is inside routers (ABRs).Each link belongs to one area.,Integrated IS-IS vs. OSPF: Area Design (Cont.),In IS-IS the area borders lie on linksEach IS-IS router belongs t
11、o exactly one area.IS-IS is more flexible when extending the backbone.,Advantages of Integrated IS-IS,Supports CLNP and IPMore extensible through TLV design,Advantages of OSPF,OSPF has more features, including:Has three area types: normal, stub, and NSSADefaults to scaled metric (IS-IS always 10)OSP
12、F is supported by many vendors.Information, examples, and experienced engineers are easier to find.,Summary,IS-IS is a popular routing protocol in the ISP industry.IS-IS is a stable, fast-converging IGP that is positioned to route IPv4, CLNS, or IPv6.All IS-IS interfaces have a default metric of 10.
13、ES-IS (for CLNS routing only) provides discovery between host and routers using hello packets to form adjacencies. Hosts send ESHs, while routers send ISHs.OSI defines routing levels 0 through 3. Level 0 is between ES and IS. Levels 1 and 2 are between IS and IS to support intradomain routing. Level
14、 3 supports interdomain routing.Level 1 is intra-area Level 2 is interarea.IS-IS and OSPF are both open-standard link-state routing protocols that support VLSM, scalability, and quick convergence.,The IS-IS Protocol,Performing IS-IS Routing Operations,OSI Addresses,OSI network layer addressing is im
15、plemented with NSAP addresses.An NSAP address identifies a system in the OSI network; an address represents an entire node, not an interface.Various NSAP formats are used in various systems, because different protocols may use different representations of NSAP.NSAP addresses are a maximum of 20 byte
16、s:Higher-order bits identify the interarea structure.Lower-order bits identify systems within area.,Integrated IS-IS NSAP Address Structure,The Cisco implementation of Integrated IS-IS distinguishes only the following three fields in the NSAP address:Area address: Variable-length field (1 to 13 octets)composed of the higher-order NSAP octets, excluding system IDand NSEL.System ID: ES or IS identifier in an area; fixed lengthof six octets in Cisco IOS software.NSEL: One octet NSAP selector, service identifier.Total length of NSAP is from 8 (minimum) to 20 octets (maximum).,
