Why a Static Route with Interface AND Numerical Next-Hop?

[ebanks2006]

Can anyone point me to documentation explaining why I'd want to use a static route like this:

ip route 10.1.2.0 255.255.255.0 Vlan100 172.16.1.1

instead of a static route like this:

ip route 10.1.2.0 255.255.255.0 172.16.1.1

I've read the IOS documentation on the "ip route" statement that addresses the issues related to proxy arp and memory/CPU exhaustion in extreme circumstances, if I were next-hopping to the interface instead of to a numerical next-hop. I grasp the benefit we get if proxy arp goes away, which can be achieved by following up an interface with a numerical next-hop in the ip route statement.

However, I can't find a good reason to static route to an interface + numerical next-hop unless you need the route to fall out of the table if the interface goes down. Is that the only benefit, or is there some other benefit I'm overlooking? It just seems like you'd static route to the numerical next-hop and call it done if we don't care what router interface the packet exits when it gets forwarded.

[network.king · Accepted solution]

Hi

Its just the benefit of overcoming the recursive lookup done by the router in case you are not specifying the next-hop interface and the interface is down.

Considering your example , just think you have a floating route and vlan 100 is down .

Router tries to do a recursive lookup for the next-hop of 172.16.1.1 eventhough the interface is down , And if there is a default route in your router , then the route will be present eventhough the interface is down and your traffic would get dropped.In case if the interface is also specified , route would go down and traffic would get routed via the floating route

ref:

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

Hope this helps

regards

vanesh k

[Richard Burts]

Ethan

I do not have right now a good link to documentation of this feature, but you have actually provided a very good explanation of the benefit of combining the interface specification along with the numerical next hop. If you configure a static route with only numerical next hop (probably an address in a connected subnet) the router will correctly route out the interface while it is up. But if the interface goes down, and if the router is able to resolve reachability through another interface (recursive route table lookup) it will forward out the different interface. But we may not want it to forward out a different interface. We may want the static route to fail. Combining the interface specification with the numeric next hop specifies that we must get to the next hop through the specified interface and if we can not do that then the static route will fail and will be withdrawn from the routing table.

And like you say if we don't care what router interface the packet exits when it gets forwarded then just code the static route with numeric next hop and call it done.

HTH

Rick

[ebanks2006]

Both responses helped a lot, and the link was exactly what I needed to share with the rest of my team. Thanks, much appreciated!