IS-IS Packet Encapsulation

It may seem unusual to begin looking at a routing protocol's packet formats and operations with a discussion of the way packets are encapsulated when being transmitted, but IS-IS is unique among the routing protocols in this respect. Rather than IS-IS packets being encapsulated into a network layer protocol, such as the Connectionless Network Protocols or the Internet Protocol (IP), IS-IS packets are encapsulated directly into data link layer, or layer 2, headers. While all other routing protocols rely on a higher layer, layer 3 or 4 of the OSI model, to deliver their data, IS-IS actually relies on its own layer 3 headers to transmit packets.

For example, when an open shortest path first (OSPF) process transmits data, ...

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