Troubleshooting Inverse-ARP on Frame-Relay

[Boscoh]

I'm a little shady on this one...so hopefully some of you guys can help me out.

If you need Inverse-ARP on a frame interface, and its not configured, what are the signs of the misconfiguration? Does the interface just not come up? Or will you encounter intermittent errors?

Specifically the situation is: one of the T1 frame-relay interface I am in charge of went down yesterday. SBC said it was because the command "no frame-relay inverse-arp IP 16" was entered on the interface. I was confused...because this T1 has been up for over a year without any problems, with that command being in it, why would it all of a sudden go down? Personally, I think they either reconfigured their equipment, and now required inverse-arp, or they told me that just to mask another problem on their network. This is their configuration, by the way...not mine. Which makes me wonder even more...if they needed it, their tech should have configured the router for it.

Am I correct in my assumptions?

 

[spidey07]

inverse arp is stricty the routers responsibility. There needs to be some way to map IP addresses to DLCIs. Inverse arp accomplshes this for you.

Think about it. What does arp do? Maps IP address to layer2 address. What is layer2 on a Frame-Relay circuit?

Frame-Relay DLCIs.

You'll get a up/up even if inverse arp isn't working. Its been so long since I've had to trouble shoot it because I've never had problems with it.

this should give you some really good info. If you use frame-relay at all I highly recommend you read it many times.

http://www.cisco.com/en/US/tech/tk713/tk237/technologies_tech_note09186a008014f8a7.shtml

 

[Boscoh]

I know what Inverse-ARP does, and why it's needed...I just dont understand why the circuit would have gone down after a year of being up with the "no frame-relay inverse-arp IP 16" command in the router. Then all of a sudden the circuit goes down and wont come back up until we re-enable inverse-arp. I'd think if inarp was the problem, we would have seen this a year ago.

So I get an up/up even if it isnt working...but wouldn't it not pass traffic if it didnt know which DLCI it was mapped to? I mean...the DLCI was mapped in the config for the interface.

EDIT:

I checked the router's current DLCI mapping...and it's 16. So they didnt change it, because thats what it was mapped to before.

So I'm back to my main point...why after a year would this connection all of a sudden decide to stop working, and SBC determine it to be because of the no inverse-arp command, when its been in there the whole time. Does anyone have an explanation?