[afnog] Router Interface Problems

[Chris Wilson]

Hi all,

On Thu, 8 Nov 2012, JC Cockburn wrote:

> i have recently encountered a problem with my router (Cisco 3700 
> series). Actually the first router (same) i was using suddenly couldn't 
> pass traffic (there was no internet) only to find out that when i 
> traceroute, traffic stuck on the inter interface (fa0/1).

What exactly do you mean by "stuck"? Did you have a routing loop, or did 
the traceroute just disappear after that point?

> Now i have backed up the configs and restore them into a new router. 

Are they current configs or old backups?

> This was working fine, but i came to realize that browsing is extremely 
> slow, and i can't even video conference because there are terribly 
> packet loss. Now i find out that there is high latency (600-800ms) from 
> the router serial interface to the modem (i'm using satellite). I logged 
> into the router, and it is strange stat i can't even ping the s0/0 of 
> the router.

If you can't ping the router from the router itself, that implies a 
misconfiguration, such as not having configured the interface with the 
right IP address. Also check your routing tables for other routes to the 
same address or subnet.

> i can ping fa0/0 and the computers inside the network (clients).
> 
> What could be the problem?  Any assistance will be highly appreciated.

Is it possible that the router is configured with the wrong IP address on 
its public interface (satellite side), or it has a route to that IP 
through another interface, so it doesn't answer ARP requests by the 
satellite modem? Then the modem might have to wait until it sees a packet 
from the router before it can learn the router's MAC address and send the 
queued, backlogged incoming traffic to the router.

Also check the IP addresses configured on the other interfaces and the ARP 
modes of all interfaces, as the same thing might be happening when 
internal hosts try to send traffic out. And try pinging the router and 
the satellite modem from local hosts, and pinging the satellite modem from 
the router.

Cheers, Chris.
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