I have a point to point T1 Serial interface on a router that is in interface UP, line protocol DOWN status. The telco has pointed to the problem being on one side of the link, where do I start looking? The router has DCD, DSR, DTR, RTS and CTS all in up status.
Start by doing a local loopback at the Telco NIU (Smartjack. On the models we have, we can press a local loopback button on the NIU and then monitor the Router Interface for UP/UP(Looped)status. If it doesn't show up/up(looped), replace all patch points, and possibly the Port Adapter Interface or WIC interface/module and try again.
You should be able to repeat this process on both ends of the link.
I also recommend building a short T-1 loopback connector you can attach directly to your serial interfaces, it will help ensure the interface is working properly and can also help determine if cross/connect - patch cabling is good/bad.
You can also ask your Telco to loop to each CSU/DSU (WIC/Router Interfaces) from their NIUs. Both local and remote looping can help identify which end of the circuit is having issues.
I had as issue yesterday where I happened upon 2 bad WIC-T1-V2 modules on a 2811 router. Telco looping help discover the issues.
We also found a bad 7206 Port adapter Serial interface that looked good in IOS, loopback, etc. The only way we confirmed a problem was to put a loopback cable right on the interface and watch it for 20-30 mins, then input errors started to accumulate and we moved the T-1 link to an open interface, moved the config, and it came up clean.
We've also had some very perplexing issues with at least 3 2811 routers in our Remote T-1 links. We start seeing alot of layer 1 errors, we call the telco and arrange an intrusive testing window. When the telco is done testing, they normalize the circuit and let us know what they found. We have had 3 instances where the link stayed up/down and we saw a lot of L1 errors. In each case, after rebooting remote 2811 routers, the links came back up clean?
I've RMA's 2 WIC-T1-V2 modules and have 2 more to send back this week. A ittle perplexing and hopefully not a new 2811 ISR issue.