Cisco 6000 Series IP DSL Switches

Field Notice: 8xDMT Line Cards and CPE Using Non-Globespan Chipset May Fail to Train in Short Loops

October 8, 2001

Products Affected


Problem Description

Users may experience a failure to train when using the following configuration:

  • 8xDMT line card (ATUC-8-DMT-1)

  • Customer premises equipment (CPE) using a non-Globespan chipset

  • Loop length shorter than 3,000 feet

This problem may be seen on one or more ports, but rarely on all ports. The affected ports are not consistent from one line card to another.


During the handshake, the ATU-C line driver is put into an inactive state (shutdown mode) while receiving information from the CPE. Due to device tolerances, the output impedance of the line driver (while in this state) varies from device to device. This variation may cause a line impedance mismatch on some ports but not on others. Affected ports vary from line card to line card.

Problem Symptoms

The problem occurs on short loops. Because it has not been seen on loops greater than 3,000 feet, field exposure may be limited. There is a higher probability of this problem occurring in lab environments during customer qualification tests using short loops.



The workaround is to add length (making the loop greater than 3,000 feet) to the line during qualification tests. Customers are advised to use loop simulation equipment or to avoid using short cables.


This issue is resolved in Cisco IOS® Release 12.2(1b)DA and later. This release includes a line driver control, which will place the line card into terminate mode instead of shutdown mode during the handshake. The device output impedance in terminate mode does not cause the line impedance mismatch.

Cisco IOS Versions Affected

All Cisco IOS Software releases earlier than 12.2(1b)DA are affected.

For More Information

If you require further assistance, or if you have any further questions regarding this field notice, please contact the Cisco Systems Technical Assistance Center (TAC) by one of the following methods: