ip unreachables — interface
ip unreachables no ip unreachables
Sending of ICMP unreachable messages for an interface
ICMP unreachable messages are generated when something about an incoming packet is unknown to the router. For example, an “ICMP host unreachable” message is generated if the router cannot deliver a package to its final destination. There are many different types of ICMP unreachable messages, and they all mean that the packet can’t be delivered for some reason. Disabling these messages can improve security because the messages can be used to discover information about your network.
The following commands prevent the router from sending IP unreachable messages through the
interface serial0 no ip unreachables