A Complete Sequence Numbers PDU (CSNP) is a complete list of the LSPs in the Designated IS‘ link state database. The CSNP contains a list of all the LSPs in the database, as well as other information that helps IS neighbors determine whether their LSP databases are in sync with one another. The Designated IS sends CSNPs to the broadcast interface. Level 1 and Level 2 each have their own Designated IS.
A Partial Sequence Numbers PDU (PSNP) is a partial list of LSPs. ISs other than the Designated IS (that is, the non-Designated ISs) send PSNPs to the broadcast interface. The CSNP interval specifies how often the Designated IS sends a CSNP to the broadcast interface. Likewise, the PSNP interval specifies how often other ISs (non-Designated ISs) send a PSNP to the broadcast interface.