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.

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 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.

Sources:

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

CCNP 1 Ver. 3.1 Kap.8