Hey, Cisco Guru's, a question!

[LordRaiden]

Hey, I got a question for all the Cisco guru's out there in ATOT. This isn't a "how do I fix it" type question. More one of curiousity.

When fixing a client router recently I encountered a really weird issue routing data. The fix was to enter the line "no ip proxy-arp" in the config and it suddenly worked.

Any ideas why this worked? It doesn't make sence to me.

 

[dmcowen674]

Originally posted by: LordRaiden
Hey, I got a question for all the Cisco guru's out there in ATOT. This isn't a "how do I fix it" type question. More one of curiousity.

When fixing a client router recently I encountered a really weird issue routing data. The fix was to enter the line "no ip proxy-arp" in the config and it suddenly worked.

Any ideas why this worked? It doesn't make sence to me.

That's odd indeed. No IP is a DNS redirector and arp refers to the old Arpanet Network.

 

[alrox]

That command disables proxy-arp.. The router was probably responding for arp queries that it shouldn't have been.

 

[LordRaiden]

Yeah, the guy who was helping me said something about getting an error that the fastethernet 0 interface was blocked for some administrative reasons or something along those lines.

I also noticed looking closer at the config, he also had to add these two lines:

no ip route-cache
no ip mroute-cache

The funny part is, we had another one sitting next to it with a much leaner config that was exactly to the letter identical to this one that worked perfectly, yet this one wouldn't work with that config. He eventually had to add those lines and the second one worked. It makes no darned sence.

 

[Tallgeese]

From Cisco: Explanation of Proxy Arp

 

[spidey07]

Depending on the router platform the commands "no ip route-cache" and "no ip mroute cache" actually severely limit performance by bypassing any kind of fast or CEF switching.

And most times you'll never need to disable proxy-arp unless you have some kind of routing muck up elsewhere in the network. So if you have to disable proxy-arp to get it to work, there is some other underlying problem.

 

[LordRaiden]

That's what I thought too. But the thing is, we couldn't find any other issues anywhere. We could get out fine to any site on the net from the router, we could get to the internal network and anywhere inside of it just fine, but you couldn't move data past the router in or out. Nothing was moving between the ethernet or the serial interface. That's what had all of us baffled. Eventually turning up the 2nd ethernet interface was what tipped us off to what was happening.

 

[spidey07]

proxy-arp allows the router to respond to arp requests on behalf of clients who are (wrongly) arping for a remote network. Routers can do this as well.

I once had to troubleshoot a network problem that shut down just about everything. Turns out it was a linksys router proxy-arping for an entire class B network.

If you had some traces you should be able to see exactly what was happening. But again I'd be concerned about a router that had route-cache disabled and needed proxy-arp disabled.

Something fishy is going on.

 

[Fatt]

I don't suppose you can post a show run, can you?

I'd just be curious.