Cisco Systems CCNA 2 manual TCP/IP Suite Control Messages, Introduction to control messages

8.2 TCP/IP Suite Control Messages

Essential Labs:	None
Optional Labs:	None
Core TIs:	None
Optional TIs:	All

Course Level Claim: Students can describe the operation of ICMP and identify the reasons, types, and format of associated error and control messages.

Hands-on skills: none

8.2.1 Introduction to control messages

ICMP is an integral part of the TCP/IP protocol suite. All IP implementations must include ICMP support for the following reasons:

•Since IP does not guarantee delivery, it has no method to inform hosts when errors occur.

•IP has no built-in method to provide information or control messages to hosts.

•ICMP is necessary to perform these functions for IP.

Explain to the students that unlike error messages, control messages are not the result of lost packets or error conditions. Instead, they are used to inform hosts of conditions such as network congestion or the existence of a better gateway to a remote network. Like all ICMP messages, ICMP control messages are encapsulated.

8.2.2 ICMP redirect/change requests

An ICMP redirect/change request can only be initiated by a gateway, which is 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. Normally there is a single gateway. In some circumstances a host can connect to a segment that has two or more directly connected routers. In these situations, the default gateway may need to use a redirect/change request to inform the host of the best path. Explain this concept with the students and make sure they understand this important process.

Default gateways only send ICMP redirect/change requests 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.

97 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 8 Copyright ♥ 2004, Cisco Systems, Inc.