Static Route age out by ARP Timeout ?

Unanswered Question
Apr 16th, 2008

Hi all,

I am curious about a simple thing :)

Lets say we got two routers connected through a hub. Attached is a simple topology. Say Router 1 has two static route entries. First pointing out to Router 2 and second to router 3. when router 2 gets disconnected. Does router 1 fails over to the second router even if its interface does not GO DOWN ? Does it check the availability of the next hop through ARP table entry for the next hop IP address ?

Regards,

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Richard Burts Wed, 04/16/2008 - 03:50

Dumlu

No the IOS will not check availability of the next hop using the ARP table entry. By default if the interface does not go down then the static route remains in the routing table and the other route will not be used. Cisco has addressed this issue with a feature called Reliable Static Routing Using Object Tracking which should give you what you want. You can use this link to find more about it:

http://www.cisco.com/en/US/docs/ios/12_3/12_3x/12_3xe/feature/guide/dbackupx.html

HTH

Rick

dumlutimuralp Wed, 04/16/2008 - 03:52

Hi Rick,

Are you sure about this ? Dont get me wrong please. Just got out of a meeting full of CCIEs and they insist that if the ARP entry ages out the route fails over to another one.

Richard Burts Wed, 04/16/2008 - 04:02

Dumlu

Yes I am sure. Lets make sure that the question that you posted here is the same as the issue that the meeting full of CCIEs was discussing. There might be some qualification in their discussion that is not in this thread or some part of the configuration that they considered that I do not know. But the default behavior of IOS is that the criteria for a static route pointing out an interface is whether the interface is up or down. The IOS does not by default check the ARP table for the next hop. This behavior is why Cisco introduced the feature that I suggested of using Object Tracking to verify a static route. Of this I am very sure.

HTH

Rick

dumlutimuralp Wed, 04/16/2008 - 04:30

Hi Rick,

thanks again. Well there were not any other depth in our conversation. just the simple topology that I sent you. They say that when the router 2 gets disconnected from the hub, router 1 automatically begins to send packets to 10.1.1.3 (router 3) ONCE THE ARP ENTRY for 10.1.1.2 AGES OUT on ROUTER 1.

regards.

Richard Burts Wed, 04/16/2008 - 04:50

Dumlu

Well I will match my CCIE against theirs and respectfully disagree with them. If router 2 gets disconnected from the hub the ARP entry may age out but router 1 will maintain the static route in its routing table pointing to router 2 and will not use the route through router 3. I suggest that you test it and let us know the outcome.

HTH

Rick

dumlutimuralp Wed, 04/16/2008 - 04:54

Hi Eric,

Thanks so much for you input.

I appreciate it. will let you know.

Regards,

Edison Ortiz Wed, 04/16/2008 - 06:21

Well I will match my CCIE against theirs and respectfully disagree with them.

I'll throw mine in there, FWIW.

I believe the discussion evolved around a hub and spoke scenario with Frame-Relay and not a hub on ethernet media.

__

Edison.

dumlutimuralp Wed, 04/16/2008 - 12:14

No it is not Edison. It was a pure point to point link , actually instead of hub, there is a metro ethernet link in between , all three nodes in the same vlan.

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