Understanding the Transmission Frequency for CSNPs on IS-IS Interfaces
The complete sequence number PDU (CSNP) interval controls the frequency at which a routing device sends a directory of its link-state database.
When IS-IS is activated on a routing device’s interface, the device first sends some IS-IS hello packets (IIHs) to its neighbors to ensure that the circuit is capable of transporting packets in both directions. In the IIHs, the router embeds information about the designated router (also called the designated intermediate system or DIS). One of the designated router roles on an IS-IS broadcast circuit is to synchronize the link-state databases on LANs. The designated router does this by periodically sending a directory of its link-state database, which is received by all the routing devices on a LAN.
If the routing device is the designated router on a LAN, IS-IS sends CSNPs every 10 seconds. If the routing device is on a point-to-point interface, it sends CSNPs every 5 seconds. The general recommendation is to use the default values or to increase the CSNP interval if there are a large number of broadcast circuits that need to be supplied with fresh CSNPs. Increasing the interval can help protect against CSNP flooding.