ip route 0.0.0.0 0.0.0.0 x.x.x.x fa0/1

Question:

Can someone explain Cisco 3560V2 Price about the below mentioned command ?

ip route 0.0.0.0 0.0.0.0 x.x.x.x(Nxt Hop IP)  fa0/1

Answer:

is there something in particular you want to know?

Static routes, in your case the default route, can be configured to point to a numerical next hop (IP address), an exit-interface, or -like your example shows- both:

ip route <network> <subnetmask> <numerical-next-hop>

ip route <network> <subnetmask> <exit-interface>

ip route <network> <subnetmask> <exit-interface> <numerical-next-hop>

The differences are explained in this document:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800ef7b2.shtml

Dinesh Kumar Mariappan:

This is command for default route.. What ever packet received to the router it simply forward to fa0/1 interface or to next IP.

This is true as long as no more specific route for the destination exists.

There are situations, where the combination of exit-int and numerical next-hop is desired.

Let me give you an example.

Lets say you have an AS and use the IP address range 192.168.0.0/16.

Your default route points to 192.168.12.2

ip route 0.0.0.0 0.0.0.0 192.168.12.2

Now imagine that there's another route to the next-hop address 192.168.12.2, e.g. a discard route (just an example):

ip route 192.168.0.0 255.255.0.0 Null0

Do you know what happens when you disable the interface connecting 192.168.12.2 (say Fa0/0)?

interface FastEthernet0/0

      ip address 192.168.12.1 255.255.255.0

      shutdown

Because of the recursive nature of the this type of routes, there's still a valid route to 192.168.12.2:

R1#show ip route 192.168.12.2

Routing entry for 192.168.0.0/16, supernet

Known via "static", distance 1, metric 0 (connected)

Routing Descriptor Blocks:

* directly connected, via Null0

R1#show ip route

S*   0.0.0.0/0 [1/0] via 192.168.12.2

S    192.168.0.0/16 is directly connected, Null0

So the link is down, however, the default-route is still there.

This behavior changes if you add the exit-interface:

ip route 0.0.0.0 0.0.0.0 FastEthernet0/0 192.168.12.2

ip route 192.168.0.0 255.255.0.0 Null0

R1#show ip route

S    192.168.0.0/16 is directly Cisco 3560V2  connected, Null0