Sequence Number PDU (SNP)

Sequence Number PDU (SNP)
Sequence number PDUs (SNP) are used primarily to ensure that routers have the most up-to-date
LSPs. If you think about it, the operation of SNPs is very similar to the use of acknowledgment
packets. There are four different types of SNPs available:
Complete sequence number PDU (CSNP) Complete sequence number PDUs (CSNPs) contain
the most up-to-date list of all LSPs. When a link first comes up, CSNPs are used to ensure
the routers have the latest LSPs to form their link-state databases. CSNPs are also used periodically
to ensure that routers have the latest information. Level 1 and Level 2 adjacencies have
their own CSNPs. In other words, a Level 1 CSNP is used only for Level 1 information, and a
Level 2 CSNP is used only for Level 2 information. CSNPs are sent out only by DISs, so you will
not find CSNPs on point-to-point connections.
Partial sequence number PDU (PSNP) A partial sequence number PDU (PSNP) contains only
the latest sequence number information for a few LSPs. Point-to-point connections do not use
CSNPs, but broadcast multi-access networks use both CSNPs and PSNPs, as outlined in the
next section, “LSP Flooding.” PSNPs can be used to request missing LSP information after
receiving CSNPs in broadcast multi-access networks. In the absence of CSNPs, point-to-point
networks use PSNPs to acknowledge the receipt of LSP routing updates. Like CSNPs, PSNPs are
also specific to the level they are representing.
Now that we have covered the different types of PDUs used in IS-IS networks, let’s take a
look at how these different PDUs are used to create a router’s link-state database.