Sequence number packets

The third category of IS-IS PDUs are sequence number packets (SNPs). SNPs are used as auxiliary mechanism and ensure integrity of the LSP-based routing information distribution process. The major task of a sequence number packet is to ensure with explicit mechanisms a level 1 or level 2 link-state database. You can realise this goal with two different packet types.

  • Complete sequence number packets (CSNP)
  • Partial sequence number packets (PSNP)

Both packet types share a similar packet format and each carries a collection of LSP summaries. The basic difference between them is that a CSNP advertised by a router contains summaries of all the known LSPs in its database, whereas the PSNP contains only a subset.

These two types of packets are used in level 1 link-state databases and in level 2 link-state databases. That means that four types of sequence number packets are used.

Complete sequence number packets (CSNP)

Most of the fields of a CSNP header are discussed in an earlier chapter.Yet untreated or very important fields are:

  • Source ID: The source ID refers to the SysID of the router that generated the CSNP. On a point-to-point link, it is the SysID of the nodes on either side of the link. On a broadcast link, it is the SysID of the designated router.
  • Start LSP ID: The start LSP ID is the LSP ID of the first LDP in the LSP entries TLV attached to the header.
  • End LSP ID: This is the LSP ID of the last LSP in the LSP entries TLV field.
  • LSP Entries: This is a collection of LSP summaries of all known LSPs in the corresponding level 1 or level 2 link-state database of the advertising router, sorted in order of ascending LSP ID.
  • Authentication Information: As defined for level 1 and level 2 LSPs. TLV type 10.

Partial sequence number packets (PSNP)

Partial sequence number packets normally have two key functions. First of all PSNPs are used by routers to acknowledge receipt of one or more LSPs over point-to-point link. The second key function is that routers request transmission of current or missing LSPs by using PSNPs. That’s true to both network types, point-to-point and broadcast links.


Abe Martey, IS-IS Network Design Solution, Kap. 3

CCNP 1 Ver. 3.1 Kap.8

sequence_number_pakete.txt · Last modified: 2009/09/13 14:37 (external edit)
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