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BSCI Exam Cram™ 2 (Exam 642-801) by Ed Tittel, Michael J. Shannon

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Database Synchronization and Routing Traffic Transport

To accomplish synchronization of the IS-IS link-state database, you use distinctive protocol data units referred to as CSNPs and PSNPs. They are often simply known as Sequence Number PDUs or SNPs. The goal of these special packets is to make sure that LSPs are exchanged in a reliable fashion. They do not actually carry the actual LSP data but rather LSP descriptors, or headers, that define the link-state packets.

Complete Sequence Number PDUs

Complete Sequence Number PDU (CSNP) packets contain an address range (a list of the LSPs held by the router), LSP ID, sequence number, checksum, and remaining lifetime value. The DIS multicasts them every 10 seconds so that the receiving ISs can compare ...

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