System information

Troubleshooting ISDN
Book Title
17-392
ISDN: Slow Performance
Symptom: ISDN connections are successfully established and communication occurs, but
performance across the link is slow.
Table 17-9 outlines the problems that might cause this symptom and describes solutions to those
problems.
Table 17-9 ISDN: Slow Performance
Possible Problem Solution
Hold queues too small
Step 1 Check for input or output drops on the ISDN interface:
• For a BRI interface, use the show interfaces bri number 12privileged exec
command
• For a PRI interface, use the show interfaces serial slot/port privileged exec
command
• For a serial interface, use the show interfaces serial number privileged exec
command
Look for drops in the command output. The output line looks similar to the
following:
output queue 0/40 0 drops; input queue 0/75 0 drops
Step 2
If there are excessive drops on the interface, use the appropriate clear
counters privileged exec command to clear the interface counters. Check for
drops on the interface again. If the values are incrementing, you should
increase the size of the input or output hold queues.
Step 3 Increase the hold queue for the interface that is dropping packets. Use the
hold-queue length out or the hold-queue length in interface configuration
command, depending on whether you are seeing output or input drops on the
interface.
Increase these queues by small increments (for instance, 25%) until you no
longer see drops in the show interfaces output.
Poor line quality
Step 1 Check for input or output errors on the ISDN interface.
• For a BRI interface, use the show interfaces bri number 1 2 privileged
exec command
• For a PRI interface, use the show interfaces serial slot/port privileged exec
command
• For a serial interface, use the show interfaces serial number privileged exec
command.
Look for errors in the command output. The output lines look similar to the
following:
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0
abort
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets
Step 2
If there are excessive errors on the interface, use the appropriate clear
counters privileged exec command to clear the interface counters. Check for
errors on the interface again. If the values are incrementing, it is probably the
result of poor line quality.
Step 3 Reduce the line speed to 56 kbps to see whether the error rate slows or stops.
Step 4 Contact your carrier to see whether something can be done to improve the line
quality. Make sure the DCE device is configured properly as well.