A common ICMP control message is the ICMP redirect/change request. This type of message can only be initiated by a gateway, which is a term commonly used to describe a router. All hosts that communicate with multiple IP networks must be configured with a default gateway. This default gateway is the address of a router port connected to the same network as the host. Figure displays a host connected to a router that has access to the Internet. Once configured with the IP address of Fa 0/0 as its default gateway, Host B uses that IP address to reach any network not directly connected to it. Normally, Host B is connected to a single gateway. However, in some circumstances, a host connects to a segment that has two or more directly connected routers. In this case, the default gateway of the host may need to use a redirect/change request to inform the host of the best path to a certain network. 

Figure shows a network where ICMP redirects would be used. Host B sends a packet to Host C on network 10.0.0.0/8. Since Host B is not directly connected to the same network, it forwards the packet to its default gateway, Router A. Router A finds the correct route to network 10.0.0.0/8 by looking into its route table. It determines that the path to the network is back out the same interface the request to forward the packet came from. It forwards the packet and sends an ICMP redirect/change request to Host B telling it to use Router B as the gateway to forward all future requests to network 10.0.0.0/8.

Default gateways only send ICMP redirect/change request messages if the following conditions are met: 

• The interface on which the packet comes into the router is the same interface on which the packet gets routed out.
• The subnet/network of the source IP address is the same subnet/network of the next-hop IP address of the routed packet.
• The datagram is not source-routed.
• The route for the redirect is not another ICMP redirect or a default route.
• The router is configured to send redirects. (By default, Cisco routers send ICMP redirects. The interface subcommand no ip redirects will disable ICMP redirects.)

The ICMP redirect/change request uses the format shown in Figure . It has an ICMP type code of 5. In addition, it has a code value of 0, 1, 2, or 3.

The Router Internet Address field in the ICMP redirect is the IP address that should be used as the default gateway for a particular network. In the example in Figure , the ICMP redirect sent from Router A to Host B would have a Router Internet Address field value of 172.16.1.200, which is the IP address of E0 on Router B.

Web Links Internet Protocol http://www.cs.panam.edu/~meng/ Course/CS6175.Protocol/ Note/ip/ip.html