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35-11
Cisco ME 3400 Ethernet Access Switch Software Configuration Guide
OL-9639-06
Chapter 35 Configuring IP Unicast Routing
Configuring IP Addressing
Routing Assistance When IP Routing is Disabled
These mechanisms allow the switch to learn about routes to other networks when it does not have IP
routing enabled:
• Proxy ARP, page 35-11
• Default Gateway, page 35-11
• ICMP Router Discovery Protocol (IRDP), page 35-11
Proxy ARP
Proxy ARP, the most common method for learning about other routes, enables an Ethernet host with no
routing information to communicate with hosts on other networks or subnets. The host assumes that all
hosts are on the same local Ethernet and that they can use ARP to learn their MAC addresses. If a switch
receives an ARP request for a host that is not on the same network as the sender, the switch evaluates
whether it has the best route to that host. If it does, it sends an ARP reply packet with its own 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 35-10. 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.
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
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.