Serial interface looped suddenly

[jharo]

We have a 7513 Cisco Router which have configured 2 Controller Cards with serial interfaces. In 3 serial links we have got configured with differents ISP's which we published our networks. Suddenly one of the serial ports was looped without reason, we check all the connection in our site and ISP site and we dont found nothing that can tell us it. After i decided reload my router and when it was ready i check the interface again and it was apparently right but no traffic signals i had. I decided use other interface with the same configuration and it works fine. Somebody knows why it happended because this port is not in use yet.

Thanks

[mljohnson]

I'm not sure what encapsulation you are using, but it's possible for there to be a loopback due to it being configured, or due to a loop on the line such that we receive our own keepalives (or LMI updates), or the loop signal could be activated on the line. There could be a HW issue (either the cable or the port), which could be the case here if you reloaded the router and it came up without any signals. But you should first ensure that the line and cable are OK before replacing the card.

[jharo]

We have checked the cable and line with our provider and all is ok. We are using HDLC for encapsulation. I think, it is a Port problem because only certain ports are not working fine in this card and using the same cable we enable other ports and they are working ok.

I appreciate your help and thanks.