Routing Assistance When Ip Routing Is Disabled - Cisco Catalyst 3550 series Software Configuration Manual

Chapter 31 Configuring IP Unicast Routing

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
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 determine 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
section on page
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:
Command
Step 1 configure terminal
Step 2
ip default-gateway ip-address
Step 3
end
Step 4
show ip redirects
Step 5 copy running-config startup-config
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
ICMP redirect cache is empty
78-11194-09
Proxy ARP, page 31-11
Default Gateway, page 31-11
ICMP Router Discovery Protocol (IRDP), page 31-12
31-10. Proxy ARP works as long as other routers support it.
Gateway
Purpose
Enter global configuration mode.
Set up a default gateway (router).
Return to privileged EXEC mode.
Display the address of the default gateway router to verify the
setting.
(Optional) Save your entries in the configuration file.
Last Use Total Uses
Catalyst 3550 Multilayer Switch Software Configuration Guide
Configuring IP Addressing on Layer 3 Interfaces
"Enable Proxy ARP"
Interface
31-11