3Com CoreBuilder 3500, Implementation Manual

Page: 294 / 592

294 C
HAPTER
11: I
NTERNET
P
ROTOCOL
(IP)
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. You can choose this default gateway 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 if 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.
Example Figure 58 shows how ICMP dynamically determines a default gateway.
Figure 58 ICMP Router Discovery
Redirected route
Intended route