The Complete Sequence NumberPDU (CSNP) is a protocol data unit (PDU) that is sent out into a network
bya specific router, the Designated Intermediate System
(DIS), and is used to maintain database synchronization.All link state PDUs (LSP) are contained in the CSNP and are stored in the link state databases (LSDB). Changes in the network topology are communicated to the LSDB
via the CSNP and updated in theLSDB
.Depending on the network topology and connection 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 connections
. In the Intermediate System to Intermediate SystemProtocol (IS-IS), the CSNP is one of four message types for determining the network topology.