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New Member

Troubleshooting Inverse-ARP on frame-relay

What are the symptoms of a frame-relay T1 that should have inverse-arp enabled, but doesnt? What exactly happens? Does traffic just not pass through the circuit?

What happened is that we have a T1 that's been working fine for the last 2 years. Last week it went down and the ISP said that it was because inverse-arp wasnt enabled on the router. I would have thought that if inverse-arp made the interface stop working then this would have happened a long time ago, seeing how it's been disabled for the last two years.

Also another question:

If you arent using subinterfaces (eg Serial0 is the frame-relay interface), does the interface default to multipoint or point to point? Can you change it, or do you have to use a subinterface?

Thanks in advance.


Re: Troubleshooting Inverse-ARP on frame-relay

Inverse-ARP maps remote IP addresses to (local) DLCIs; its operation compares pretty well with IP ARP. If you're using a multipoint interface and don't have static DLCI->IP mappings defined, inverse-ARP is how you know which DLCI to use to reach a given remote IP. Offhand, the only way I can see such a link working with inverse-ARP disabled is if the other side has it enabled, and is making ARP requests to you that your router then caches. But it doesn't seem proper to me for a router to maintain an inverse-ARP cache if inverse-ARP is disabled, so I don't know how plausible of an explanation this is.

In any case, Frame Relay interfaces default to multipoint. I don't believe you can make a parent interface point-to-point, but I can't say that I've tried it.

New Member

Re: Troubleshooting Inverse-ARP on frame-relay

I would also question your ISPs explanation. I don't think a T1 would actually drop without inverse-APR. You would probably just not be able to use it as an inverse-ARP problem would create an address reachablilty problem.

If your router is not using Inverse ARP, this means that you have a frame-relay map statement configured. So it's not like your router doesn't know the address of the other side of the link or which DLCI to use.

Physical interfaces are multipoint by default. I think if you wanted to use a poing-to-point, you would need to configure a p-to-p subinterface.

Good luck,