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IS-IS Network Design Solutions by Scott Sturgess, Abe Martey

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IS-IS Areas and Routing Hierarchies

As specified in ISO 10589 and RFC 1195, the IS-IS protocol supports a two-level hierarchy for managing and scaling routing in large networks. A network domain can be carved out in a planned way or arbitrarily by the network designer or architect into small segments known as areas. This allows hierarchical routing to be leveraged for efficient routing within the domain. Integrated IS-IS uses the legacy CLNP node-based addresses to identify routers even in pure IP environments. The CLNP addresses, which are known as Network Service Access Points (NSAPs), are made up of three components: an area identifier (area ID) prefix, followed by a system identifier (SysID), and an N-selector. The N-selector refers to the ...

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