IS-IS: CSNP and PSNP Packets

Complete Sequence Number PDUs are similar to OSPF DBD packets.  The purpose of these types of packets is to advertise a complete list of LSPs in the senders LSDB.  Any neighbor who receives these CSNP packets can compare their existing LSDB to the contents of the CSNP, if any LSPs have higher sequence numbers in the CSNP then the neighbor knows it needs to request an update of the LSP to update its LSDB to the most recent version or get any LSP it may not already be aware of.

CSNP packets advertise all the LSPs from the sender in Ascending order starting with 0000.0000.0000.00-00 and ending at FFFF.FFFF.FFFF.FF-FF.  If there are so many LSPs that a CSNP needs to be broken up, it will do so with as many LSP headers that can fit, and pick up where it left off on subsequent CSNP packets.

Point to point links exchange CSNP packets during initial adjacency buildup.

Broadcast networks have CSNP packets sent at regular intervals by the DIS.

Partial Sequence Number Packets are used by IS-IS routers to request updated LSPs from neighbors and acknowledge that they have received them.  a single PSNP can request multiple LSPs.



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