I'm Peter working in an ISP network operations. I have problems understanding this alarm in Cisco GSR. July 15 04:44:55 EDT: %SONET-4-ALARM: POS5/0/0: B3 BER exceeds threshold, TC alarm declared. How do I troubleshoot this problem? The interface is up but the line protocol went down because of it. What we did was we requested a loop from our international peering facing our side and we were able to see the loop. After releasing the loop and normalizing the line, line protocol went up and traffic started to flow. What's the first thing to do in problems like this?