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