Cisco Systems 3560 Manual De Usuario

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 

. 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 
listen to Routing Information Protocol (RIP) 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.

Enter global configuration mode.

ip default-gateway ip-address

Set up a default gateway (router).

end 

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.