Cisco Systems ME 3400 Routing Assistance When IP Routing is Disabled

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, 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.

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.

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.