System information

Troubleshooting Serial Line Problems 15-337
CSU and DSU Loopback Tests
CSU and DSU Local Loopback Tests for HDLC or PPP Links
Following is a general procedure for performing loopback tests in conjunction with built-in system
diagnostic capabilities:
Step 1 Place the CSU/DSU in local loop mode (refer to your vendor documentation). In local
loop mode, the use of the line clock (from the T1 service) is terminated, and the DSU is
forced to use the local clock.
Step 2 Use the show interfaces serial exec command to determine whether the line status
changes from “line protocol is down” to “line protocol is up (looped),” or if it remains
down.
Step 3 If the line protocol comes up when the CSU or DSU is in local loopback mode, this
suggests that the problem is occurring on the remote end of the serial connection. If the
status line does not change state, there is a possible problem in the router, connecting
cable, or CSU/DSU.
Step 4 If the problem appears to be local, use the debug serial interface privileged exec
command.
Step 5 Take the CSU/DSU out of local loop mode. When the line protocol is down, the debug
serial interface command output will indicate that keepalive counters are not
incrementing.
Step 6 Place the CSU/DSU in local loop mode again. This should cause the keepalive packets to
begin to increment. Specifically, the values for mineseen and yourseen keepalives will
increment every 10 seconds. This information will appear in the debug serial interface
output.
If the keepalives do not increment, there may be a timing problem on the interface card
or on the network. For information on correcting timing problems, refer to the section
“Troubleshooting Clocking Problems,” earlier in this chapter.
Step 7 Check the local router and CSU/DSU hardware, and any attached cables. Make certain
the cables are within the recommended lengths (no more than 50 feet [15.24 meters], or
25 feet [7.62 meters] for a T1 link). Make certain the cables are attached to the proper
ports. Swap faulty equipment as necessary.
Figure 15-10 shows the output from the debug serial interface command for an HDLC serial
connection, with missed keepalives causing the line to go down and the interface to reset.