Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 
Community Member

How do you know if it's the T1 line or Router that is the problem?


I'm very new to T1's and routers. Lately our router seem to go down, I'm not sure how to first go about troubleshooting. I do not want to go to the vendor if it's the router. We are using a Cisco 2501 router, and it randomlys goes down. I can't ping the router when it goes down. The only thing I can do is turn it off and on to get it going again.

The LED on the router is on and green. But there's no sign of packets being received or transmitted on the CentreCOM MX20T MAU unit. Any info or tips you can give me is greatly appreciated.


Community Member

Re: How do you know if it's the T1 line or Router that is the pr

When you say that you can't ping the router it doesn't help much. Are you pinging the serial interface? Are you local or remote? Anyway I will try to help.

The things to consider are the router, the serial cable (V.35?), the CSU/DSU, the T1 line, the remote router, the remote serial cable, or the remote CSU/DSU. Since you say that a reboot will return it to normal, I will guess that it is not the remote equipment. We can look into that if we can't find the trouble locally.

A bad T1 line can cause the CSU/DSU to go down and thus the router interface will be down, but rebooting the router will not fix a bad T1, so lets skip that for now.

So far, we have used theory to sectionalize the trouble to the local router, the local serial cable, or the local CSU/DSU. How to troubleshoot: When it goes down, what is the condition of the interface? Did you issue the 'show interface' command? Did it say 'up' and 'up' for the physical interface and line protocol? That is good. 'up' and 'down' or 'down' and 'down' are bad.

I'm going to guess that the conditions were 'down' and 'down'. What were the conditions of the control lead (shown at the bottom of the "show interface" output)? Any leads that are down will lead you to the trouble. DTE, and RTS up mean that the router is ready to go. DSR, DCD, and CTS mean that the CSU/DSU is ready to go.

What does the CSU/DSU tell you? Most have LEDs to indicate the condition of at least some of the control leads.

When the line goes down you can sectionalize further as follows: Unplug the T1 line from the CSU and plug in a looping plug. Did it come up? Yes = T1 line trouble. No = keep looking. Next, do a local loopback in the CSU/DSU. Most of them will do this either via a switch or via a menu selection. Did it come up? Yes = CSU/DSU trouble. No = keep looking. Next check that the cable between the router and CSU/DSU is good and that it is securely connected. If all of these steps are followed and the trouble still points to the router, you can switch interfaces on the router. Just copy your config to the other interface and move the serial cable.

Try these steps and let us all know what you find. If you haven't fixed it we can try some more things. Good luck!!!

Community Member

Re: How do you know if it's the T1 line or Router that is the pr

Thanks Michael,

As I said, this is my first ever attempt at troubleshooting router/line. Our IT manager is always in our main office, so I'm just the other tech here at this office trying to figure out why we lose Internet connection at random times.

I've nevered worked with the CSU/DSU unit before, but I believe it's the unit connected to our router to the telco and their CSU/DSU? I'll have to find out more info on how to use the unit (AdTran TSU 120) as well.

I can't ping our local router. I will continue with your suggestions, and hopefully I will come across a solution. Thanks for all the suggestions, please keep checking the forum for my progress. Thanks again.

CreatePlease to create content