807
Configuring IP Unicast Routing
Configuring IP Addressing
EXAMPLE
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface gi0/2
Switch(config-if)# ip proxy-arp
Switch(config-if)# end
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 807
Default Gateway, page 807
ICMP Router Discovery Protocol (IRDP), page 808
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 Enabling Proxy ARP, page 806. 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.
DETAILED STEPS
Use the no ip default-gateway global configuration command to disable this function.
EXAMPLE
Switch# configure terminal
Command Purpose
1. configure terminal Enter global configuration mode.
2. ip default-gateway ip-address Set up a default gateway (router).
3. end Return to privileged EXEC mode.
4. show ip redirects Display the address of the default gateway router to verify
the setting.
5. copy running-config startup-config (Optional) Save your entries in the configuration file.
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