Internet Control Message Protocol

Internet Control Message Protocol (ICMP) assists the operation of the IP network by delivering messages about the network’s functionality—or lack thereof. ICMP includes functions for the following:
...Communicating network errors—Such as host or network unreachable.
...Announcing network congestion—An example is the ICMP Source Quench messages used to cause a sender to slow down transmission because of a router buffering too many packets.
...Provide troubleshooting tools—The Echo function is used by the ping utility to test connectivity between two systems.
...Communicate timeouts in the network—If a packet’s TTL reaches 0, an ICMP message can be sent announcing this fact.

ICMP protocol unreachable messages

If the Cisco device receives a nonbroadcast packet destined for itself that uses an unknown protocol, it sends an ICMP protocol unreachable message back to the source. Similarly, if the device receives a packet that it is unable to deliver to the ultimate destination because it knows of no route to the destination address, it sends an ICMP host unreachable message to the source. This feature is enabled by default. To enable it if it’s disabled, use the following command:

Router(config-if)# ip unreachables


ICMP redirects
If the router resends a packet through the same interface on which it was received, the Cisco IOS Software sends an ICMP redirect message to the originator of the packet, telling the originator that the router is on a subnet directly connected to the receiving device and that it must forward the packet to another system on the same subnet. To enable the sending of ICMP redirect messages if this feature was disabled, use the following command:

Router(config-if)# ip redirects