Network Warrior, 2nd Edition

Troubleshooting Frame Relay

Troubleshooting Frame Relay is quite simple once you understand how it works. Remember that most of the information regarding PVCs is delivered from the DCE device, which is usually the telecom switch on the far end of your physical link.

The key to any troubleshooting process is problem isolation. You need to identify where the problem lies so you can determine a corrective course of action. Follow these steps, and you’ll quickly isolate where the trouble is:

Physical layer first!

Is the cable plugged in? Is the cable a known good cable? Is the cable on the other end plugged in? This may sound silly, but you’ll feel pretty foolish if you call Cisco for help only to find that the cause of your woes was an unplugged cable.

Is the serial link up?

Make sure your serial link is up via a show interface. Leaving an interface in a shut-down state has a tendency to prevent traffic from being passed over it.

Are you receiving LMI?

Remember that LMI is sent from your locally connected telecom device. If you’re not receiving LMI, you’re not getting status messages regarding your VCs, so the router will not know that they exist. There are a couple of ways to see whether you’re receiving LMI:

show interface

The output from a show interface command for a Frame Relay–encapsulated interface will include LMI counters. LMI updates are received every 10 seconds, so executing the command and then waiting 10 seconds or more and executing the command again should show an increase in ...