System information

Troubleshooting Serial Line Problems 15-323

Using the show controllers Command

The show controllers exec command is another important diagnostic tool when troubleshooting

serial lines. The command syntax varies depending on platform:

• For serial interfaces on Cisco 7000 series routers, use the show controllers cbus exec command

• For Cisco access products, use the show controllers exec command

• For the AGS, CGS, and MGS, use the show controllers mci exec command

Figure 15-2 shows the output from the show controllers cbus exec command. This command is

used on Cisco 7000 series routers with the Fast Serial Interface Processor (FSIP) card. Check the

command output to make certain that the cable to the channel service unit/digital service unit

(CSU/DSU) is attached to the proper interface. You can also check the microcode version to see

whether it is current.

Possible Problem Solution

The following problems can result in

this symptom:

• Line interruptions due to an

external source (such as physical

separation of cabling, red or

yellow T1 alarms, or lightning

striking somewhere along the

network)

• Faulty switch, DSU, or router

hardware

Step 1 Check hardware at both ends of the link (attach a breakout box or a

serial analyzer and test to determine source of problems).

Step 2 If an analyzer or breakout box is unable to identify any external

problems, check the router hardware.

Step 3 Swap faulty equipment as necessary.