There are 2 sites,the central and the remote site,both are 3660 routers.There are 3 T1s going between them.The Third T1 always shows a serial interface up and line protocol down.When the T1 actually goes down,it comes back up only after rebooting the central site router.But still the line protocol shows down.It runs fine for about a month and then again the T1 goes down.We have to reboot the central router to get everything back up.The clocking,PPP and serial line options are all configured correctly.The Service provider has also verified the circuit and there are no problems.Also,the router always shows an "LCP closed " in the sh interface output.Any thoughts why the interface is always having a line protocol down status whether the T1 is up or down.
Try to turn on "keepalive" between all those T1's. Also try to swap the T1's between ports and see if the problem follows with 3rd T1 or it follows with port on the router. That way we can verify the possible hardware issue.
Don't rule out a circuit issue even though the service provider has said things are good. Can the service provider loop both internal DSU's when the problem occurs? If they cannot, can they loop both Dmarks at each end? Swapping the lines, as suggested in another reply, would help isolate who has the problem.
I have experience the same exact problem on my 7206VXR. I was having this problem with IOS 12.2.1d on both end of the sites. I upgraded to IOS 12.2.7c and it got rid of the problem. I hope this helps you.
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