This holds the Protocol version reported in the PDU we received. This notification-only object captures the IS-IS protocol version number the offending PDU declared, providing direct evidence for a version-incompatibility event without separate packet capture, since ciiSysVersion on virtually every modern implementation reads one and a PDU reporting anything else flags a nonstandard implementation or corrupted PDU. For example, a trap carrying this object with a value other than 1 would immediately point the admin toward investigating whatever unusual or non-compliant device sent that particular PDU.