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.
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:
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