complete sequence number PDU (CSNP)

The Complete Sequence Number PDU (CSNP) is a protocol data unit (PDU) sent into a network by a specific router, the Designated Intermediate System( DIS), and is used to maintain database synchronization.

All link state PDUs( LSP) are containedin the CSNP and are stored in the link state databases( LSDB). Changes in the network topology are communicated to and updated in the LSDB via the CSNP.

Depending on the network topology and link configuration, the CSNP is sent out at regular intervals, for example in broadcast configurations or in OSPF networks(Open Shortest Path First), or only once, for example in point-to-point links.

In the Intermediate System to Intermediate System Protocol( IS-IS), the CSNP is one of four message types for determining the network topology.