The system interface is associated with the network entity (such as, a specific router or switch), not a specific interface. The system interface is also referred to as the loop-back address.
The system interface is used to preserve connectivity (when routing re-convergence is possible) when an interface fails or is removed. The system interface is also referred to as the loop-back address and is used as the router identifier. A system interface must have an IP address with a 32-bit subnet mask.