Cisco Systems CCNA 2 manual Router solicitation message, Congestion and flow control messages

process begins when the host sends a multicast router solicitation message to all routers with the address 224.0.0.2. If a router solicitation message is sent to a router that does not support the discovery process, the solicitation will go unanswered. However, if it is supported, a router advertisement is sent in return.

8.2.7 Router solicitation message

A host will generate an ICMP router solicitation message in response to a missing default gateway. This message is sent multicast. This is the first step in the router discovery process. A local router will respond with a router advertisement that identifies the default gateway for the local host.

8.2.8 Congestion and flow control messages

Congestion occurs when multiple computers try to access the same receiver or when traffic from a high speed LAN reaches a slower WAN connection. The effect of congestion on a network is dropped packets that result in a loss of data. To reduce data lost, ICMP messages must be sent to the source of the congestion. This type of ICMP message is called a source- quench message. The source-quench message notifies the sender of the congestion and asks the sender to reduce its rate. This usually reduces the congestion. The rate of transmission will slowly increase if no other source-quench messages are received. One way ICMP source- quench messages might be used effectively is in a SOHO. Develop an example of network congestion. Have the students come up with their own ideas about the factors that cause network congestion.

99 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 8 Copyright ♥ 2004, Cisco Systems, Inc.