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IS-IS: Deployment in IP Networks by Alvaro Retana, Russ White

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Exchanging Databases

Once a new neighbor is discovered, the intermediate systems must exchange their databases. To exchange databases, four special types of packets are used: level 1 and level 2 link state packets, complete sequence number packets (CSNPs), and partial sequence number packets (PSNPs). Each of these packet types is described below.

Level 1 and Level 2 Link State Packets

While ISO 10589 defines separate level 1 and level 2 link state information packet types, they are similar enough in structure that we are going to cover both types together, with just the differences between them noted below. Level 1 link state packets are type 18, and level 2 link state packets are type 20. In addition to the information contained in the common ...

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