Troubleshooting Cisco WAN Lines
Common WAN Issues fall into these categories:
· Circuit is down
· Circuit is in loop
· Circuit is taking errors
WAN interface problems can be identified by the status line of the show interfaces serial display. See table below.
Serial Lines: show interfaces serial Status Line Conditions
Status Line Condition | Possible Problem | Solution |
Serial x is up, line protocol is up | None | This is the proper status line condition. No action is required |
Serial x is up, line protocol is up (looped)
| A loop exists in the circuit. The sequence number in the keepalive packet changes to a random number when a loop is initially detected. If the same random number is returned over the link, a loop exists. | Reset the CSU/DSU, and inspect the line status. If the line protocol comes up, no other action is needed. If not, contact the leased-line or other carrier service for line troubleshooting assistance. |
Serial x is up, line protocol is down | A high error rate has occurred due to a telephone company service problem.
A CSU/DSU or Router (interface) hardware problem has occurred | Loop CSU/DSU (DTE loop). If the problem continues, it is likely that there is a hardware problem. Swap out bad hardware; (CSU/DSU, cabling, router port) If the problem does not continue, it is likely that there is a telephone company problem. |
Serial x is down, line protocol is down | The router is not sensing a carrier signal (that is, the CD is not active). A telephone company problem has occurred—line is down or is not connected to CSU/DSU. Cabling is faulty or incorrect. Hardware failure has occurred (CSU/DSU). | Check the LEDs on the CSU/DSU to see whether the CD is active. Verify that you are using the proper cable and interface. Contact your leased-line or other carrier service to see whether there is a problem. Swap faulty parts. If you suspect faulty router hardware, change the serial line to another port. If the connection comes up, the previously connected interface has a problem. |
CSU and DSU Loopback Tests
If the output of the show interfaces serial exec command indicates that the serial line is up but the line protocol is down, use the CSU/DSU loopback tests to determine the source of the problem. Perform the local loop test first, and then perform the remote test
The use of extended ping tests can be a great method to generate traffic & verify condition of the circuit.
Step 1 Put the CSU/DSU into local loopback mode.
Step 2 Configure the extended ping command to send different data patterns and packet sizes. (all-zeros 1500-byte ping and an all-ones 1500-byte ping) respectively.
Step 3 Examine the show interfaces serial command output and determine whether input errors have increased. If input errors have not increased, the local hardware (DSU, cable, router interface card) is probably in good condition. Assuming that this test sequence was prompted by the appearance of a large number of CRC and framing errors, a clocking problem is likely.
Step 4 If you determine that the clocking configuration is correct and is operating properly, put the CSU/DSU into remote loopback mode.
Step 5 Repeat the ping test and look for changes in the input error statistics.
Step 6 If input errors increase, there is a problem either in the serial line or on the CSU/DSU. Contact the WAN service provider and troubleshoot as necessary.
Ciscosteps# ping
Protocol [ip]:
Target IP address: 10.10.10.1
Repeat count [5]: 1000
Datagram size [100]: 1500
Timeout in seconds [2]:
Extended commands [n]: yes
Source address:
Type of service [0]:
Set DF bit in IP header? [no]:
Validate reply data? [no]:
Data pattern [0xABCD]: 0x0000
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]:
Data pattern Options:
- all zero’s: 0x0000
- all one’s: 0xffff
Show Interfaces Serial Field Descriptions (continued)
