Internet Control Message Protocol

Internet Control Message Protocol (ICMP) is a network layer (Layer 3) Internet protocol that reports errors and provides other information relevant to IP packet processing. ICMP is fully documented in RFC 792. ICMP's purpose is to report error and control messages.

ICMP provides a number of useful services supported by the TCP/IP protocol, including ping requests and replies. Ping requests and replies enable an administrator to test connectivity with a remote device.

Be aware that ICMP runs over IP, which means that there is no guarantee of delivery (because IP is a connectionless protocol). Example 2-9 provides a sample ping command in which an administrator wants to see if a remote device is reachable by sending the remote device a ping request from a Cisco router. By default, a Cisco router will send out a series of five ICMP requests whenever the ping command is issued. Example 2-9 displays a sample ping request to the remote IP address on Router R2. Example 2-9 ping>ping

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 131.10


Success rate is 100 percent (5/5),


The ping command has a number of reporting mechanisms that run over ICMP. The exclamation point (!) indicates a successful reply. The ping command can also advise you, using a special code character, that the end device is not reachable, as depicted in Table 2-6.

Table 2-6 Possible Test Characters When Using the ping Command




Each exclamation point indicates the receipt of a reply.

Each period indicates that the network server timed out while waiting for a reply.


Destination unreachable.


Network unreachable.


Protocol unreachable.


Source quench.


Could not fragment.


Unknown packet type.

NOTE Cisco IOS provides a detailed version of the ping tool, which you can evoke by typing ping in the enabled mode. This command is known as the extended ping command.

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