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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 22 Configuring IP Unicast Routing
Configuring IP Addressing
Ethernet MAC address, and the host that sent the request sends the packet to the switch, which forwards
it to the intended host. Proxy ARP treats all networks as if they are local and performs ARP requests for
every IP address.
Proxy ARP is enabled by default. To enable it after it has been disabled, see the
“Enable Proxy ARP”
section on page 22-13
. Proxy ARP works as long as other routers support it.
Default Gateway
Another method for locating routes is to define a default router or default gateway. All nonlocal packets
are sent to this router, which either routes them appropriately or sends an IP Control Message Protocol
(ICMP) redirect message back, defining which local router the host should use. The switch caches the
redirect messages and forwards each packet as efficiently as possible. A limitation of this method is that
there is no means of detecting when the default router has gone down or is unavailable.
Beginning in privileged EXEC mode, follow these steps to define a default gateway (router) when IP
routing is disabled:
Use the no ip default-gateway global configuration command to disable this function.
This example shows how to set and verify a default gateway:
Switch(config)# ip default-gateway 10.1.5.59
Switch(config)# end
Switch# show ip redirect
Default gateway is 10.1.5.59
Host Gateway Last Use Total Uses Interface
ICMP redirect cache is empty
ICMP Router Discovery Protocol (IRDP)
Router discovery allows the switch to dynamically learn about routes to other networks using IRDP.
IRDP allows hosts to locate routers. When operating as a client, the switch generates router discovery
packets. When operating as a host, the switch receives router discovery packets. The switch can also
listen to Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP) routing
updates and use this information to infer locations of routers. The switch does not actually store the
routing tables sent by routing devices; it merely keeps track of which systems are sending the data. The
advantage of using IRDP is that it allows each router to specify both a priority and the time after which
a device is assumed to be down if no further packets are received.
Each device discovered becomes a candidate for the default router, and a new highest-priority router is
selected when a higher priority router is discovered, when the current default router is declared down,
or when a TCP connection is about to time out because of excessive retransmissions.
Command
Purpose
Step 1
configure terminal
Enter global configuration mode.
Step 2
ip default-gateway ip-address
Set up a default gateway (router).
Step 3
end
Return to privileged EXEC mode.
Step 4
show ip redirects
Display the address of the default gateway router to verify the
setting.
Step 5
copy running-config startup-config
(Optional) Save your entries in the configuration file.
