This is the value of ifIndex for the interface to which this circuit corresponds, and this object cannot be modified after creation. It binds an IS-IS circuit to a specific physical or logical interface in the standard ifTable, and because the MIB forbids changing it after row creation, an admin who wants to move IS-IS from one interface to another must delete the circuit entry and create a new one. For example, to move IS-IS off a retired Gi0/1 (ifIndex 5) onto its replacement Gi0/5 (ifIndex 20), the admin must destroy the existing circuit row and create an entirely new circuit row bound to ifIndex 20.