TAILIEUCHUNG - The Complete IS-IS Routing Protocol- P5

The Complete IS-IS Routing Protocol- P5:IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. Thus, if one is at all interested in routing, it is well worth the time and effort to learn IS-IS. | Names System- LAN- and LSP-IDs 107 System-ID Pseudonode- Fragment- ID ID Figure . The LSP-ID uniquely identifies an IS-IS router announcement - the first 6 bytes represent the System-ID of the sender therefore the CLI renders the output using the Hostname-to-System-ID database The System-ID field of the LSP-ID is displayed as a name. However the origin router s System-ID is displayed with the show isis hostname command the same in IOS and JUNOS which displays the hostname cache on the local router. IOS command IOS marks the local node with an asterisk Frankfurt show isis hostname Level System ID .J Dynamic Hostname London Frankfurt Washington JUNOS command JUNOS displays in addition if the entry has been learned via other routers or if it has been locally configured. The local node is always marked Static . hannes@London show isis hostname IS-IS hostname database System ID . Hostname London Frankfurt Washington Type Static Dynamic Dynamic Summary This chapter explored the foundations of IS-IS. The independence of area addressing and routing hierarchy was contrasted to the OSPF model where Area 0 implicitly makes up a 108 4. IS-IS Basics routing hierarchy. The concept of an arbitrarily assigned level to the underlying physical topology was explained. This flexibility allows IS-IS to make very resilient POP topology without spending extra costs for physical intra-POP links just to heal the topology. The IP addressing model and the OSI addressing model were discussed in a comparative way interestingly the IS-IS model corresponds almost exactly to the unnumbered IP routing model. IS-IS inherits its addressing structure from the OSI suite of protocols. Address assignment is a relatively easy task. The fixed part of the NET can be calculated based on the IP loopback address of the router and or the POP topology codes that are unique to each service .

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