6.2. IS-IS Database Synchronization
Whereas OSPF requires neighbors to explicitly agree to begin a database-exchange process and relies on a strict state machine to manage the process, IS-IS relies on a simple process in which a router periodically describes its complete database to its neighbors.
 In reality, ISO 10589 does not require periodic database descriptions on point-to-point links, although some implementations do it as an optimization.
On point-to-point links, neighbors exchange CSNPs to describe the contents of their databases. If a router sees in a received CSNP an unknown LSP or more recent copy of a known LSP, it sends a PSNP to its neighbor requesting a copy of the LSP. Likewise, if the router sees in the received CSNP that ...