Understanding Checksums on IS-IS Interfaces for Error Checking
The checksum enables IS-IS to check at the receiver if the IS-IS protocol frames have become corrupted while traversing the network.
Sometimes corrupt IS-IS protocol frames can go undetected. If routing control traffic becomes corrupted, it is likely that user payload traffic might be corrupted, too. This can lead to unacceptable throughput. To prevent corrupt frames from going undetected, we recommend enabling checksumming on the IS-IS interfaces.
To review, IS-IS hello (IIH) PDUs establish adjacencies with other routing devices. A partial sequence number PDU (PSNP) is used by an IS-IS router to request link-state PDU information from a neighboring router. The complete sequence number PDU (CSNP) lists all the link-state PDUs in the link-state database.
The original specification for IS-IS does not provide checksums for IIHs, CSNPs, and PSNPs.
RFC 3358, Optional Checksums in Intermediate System to Intermediate System (ISIS) introduced an optional type, length, and value (TLV) tuple that provides checksums for IIHs, PSNPs, and CSNPs.
Junos OS supports the checksumming TLV on a per-interface basis.