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Internet Control Message Protocol (ICMP)
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ICMP performs these tasks:
■
Creates more efficient routing ( ICMP Redirect ) — Often the host
route configuration specifies the minimum possible routing data that
is needed to communicate (for example, the address of a single
router). The host relies on routers to update its routing table. In the
process of routing packets, a router may detect that a host is not using
the best route. The router sends an ICMP Redirect to this host,
requesting that the host use a different gateway when it sends
packets to a destination. The host then sends packets to that
destination using the new route if it is able to interpret ICMP Redirect
directives.
■
Uses the router with the highest preference level as the default
gateway (
ICMP Router Discovery ) — ICMP Router Discovery is
useful if you have multiple gateways that connect a particular subnet
to outside networks. By using the preference setting, you can select
which gateway is the preferred choice.
For more information about ICMP Redirect and ICMP Router Discovery,
see “Internet Control Message Protocol (ICMP)” and “ICMP Router
Discovery” later in this chapter.
ICMP Router
Discovery
ICMP Router Discovery directs a host to use the router with the highest
preference level as the default gateway. ICMP does this by enabling hosts
that are attached to multicast or broadcast networks to discover the IP
addresses of their neighboring routers and determine which router to use
for a default gateway. If you prefer, you can make this default gateway
choice yourself.
Important
Considerations
Keep the following points in mind with ICMP Router Discovery:
■
You need not manually configure a default route.
Although IP traffic may initially be directed to any of the routers on the
LAN, ICMP Redirect messages subsequently channel IP traffic to the
correct router.
■
ICMP Router Discovery is useful on large networks, or when the
network topology has undergone a recent change.
■
If you are on a small network that is relatively stable, consider using a
static route to the gateway instead of ICMP Router Discovery to
reduce network traffic.