IS-IS is a link-state protocol that relies on the rapid and consistent flooding of Link-State PDUs (LSPs) to ensure that every router in an area has an identical view of the topology. To manage the 'freshness' of information, IS-IS uses a Sequence Number---a 32-bit unsigned integer that increments every time the originating router makes a change to its LSP.

According to Juniper Networks technical documentation, when a router receives an LSP, it performs a comparison between the received LSP and the version it currently holds in its Link-State Database (LSDB). If the received LSP has a higher sequence number, the router concludes that this is 'newer' and more accurate information. The router will then perform two immediate actions:

Update: It replaces the older LSP in its LSDB with the newly received version.

Flood: It propagates the new LSP to all other neighbors (except the one that sent it) to ensure the entire area converges on the new data.

If the sequence numbers were equal, the router would ignore the incoming PDU as it already has the information. If the received sequence number were lower, the router would conclude its own database is more recent and would actually send its own 'newer' version back to the neighbor to bring them up to date (a process called 'poisoning' or refreshing the neighbor). Complete Sequence Number PDUs (CSNPs) (Option C) are used during initial database synchronization or periodic checks on broadcast links, but the primary response to a 'newer' LSP is immediate database update and flooding.