If an interface fails or is disabled a router MUST remove and stop advertising all routes in its forwarding database that make use of that interface. It MUST disable all static routes that make use of that interface. If other routes to the same destination and TOS are learned or remembered by the router, the router MUST choose the best alternate, and add it to its forwarding database. The router SHOULD send ICMP destination unreachable or ICMP redirect messages, as appropriate, in reply to all packets that it is unable to forward due to the interface being unavailable.