Cisco WAN Switching Modules

Document ID: 10872

Contents

Introduction
Prerequisites
      Requirements
      Components Used
      Conventions
Error Definition
Error Example
Troubleshooting
Related Information

Introduction

This document provides troubleshooting information for broadband trunk module (BTM) Tx TS Pkt Drp errors.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

Conventions

For more information on document conventions, refer to the Cisco Technical Tips Conventions.

Error Definition

Tx TS Pkt Drp errors apply to the IGX BTM with E1, E2, E3, and T3 backcards.

BTM packet drop errors indicate the number of cells discarded from these trunk queues:

Causes for Tx TS Pkt Drp errors include:

• Issue the SuperUser-level command dsptrkutl to check for high trunk utilization. Packet drops can occur when trunk utilization reaches 85 percent. This situation most likely occurs if the utilization of connections was underestimated during configuration.

  For statistical real-time services such as low-speed data connections with data frame multiplexing (DFM), trunk bandwidth is assigned to each connection based on the connection configuration. DFM uses a repetitive, pattern-suppressing algorithm in order to provide data compression.

  If the assumption about how much bandwidth DFM can save was too optimistic, then connections can generate more FastPackets than the trunk can handle. BTM queue overflow can result in Tx TS Pkt Drp errors on the trunk.

  Because actual fill grade can vary quickly for the time-stamped (TS) queue, and because the rate of FastPackets per connection depends on traffic patterns of the end system, packet drops on trunk queues typically do not follow predictable patterns. However, more drops occur in peak hours than in off-peak hours.

  If a small number of packet drops occurs, monitor the trunk for a few days. Take action only if discards reduce service quality. If this occurs, the most likely symptom is end-system retransmission or timeouts.

• Misconfigured trunk parameters as indicated in the SuperUser-level command cnftrkparm.

Error Example

The likely location of equipment errors is highlighted in yellow here:

Troubleshooting

Use this procedure to troubleshoot the Tx TS Pkt Drp errors:

1. Issue the SuperUser-level dsptrkutl command to check current trunk utilization.

   1. Issue the clrtrkerrs command frequently to clear trunk error statistics.

   2. When dsptrkerrs shows dropped packets, issue the dsptrkutl command to determine current trunk utilization. If the Peak Interval Utilization field is higher than 85 percent, then queue overflows are responsible for the packet drops.

   3. Verify the trunk errors and utilization at the distant end with these two commands:

      • Issue the dsptrks command to identify the distant-end switch and trunk number.

      • Issue the vt command to open a virtual session with the distant-end switch.

   4. For immediate relief, route voice connections over alternate trunks.

   5. Issue the SuperUser-level dsptrkcons command to identify the total number of connections routed over the problem trunk.

   6. Issue the dsprts command to identify the connection identifiers and the current route for all connections routed across the problem trunk.

   7. Issue the these SuperUser-level commands in order to display utilization for each connection routed across the problem trunk.

      • dsputl—Voice connection

      • dspdutl—Data connection

      • dspchstats—ATM or Frame Relay connection

2. To resolve the problem, evaluate the network design of connection parameters and correct if necessary. For data connections with DFM, the parameter that affects trunk bandwidth allocation is utilization.

   1. Issue the dspcon and dspchcnf commands to verify connection settings.

   2. To change utilization, issue the cnfchutl command.

      The lower the configured utilization for a connection, the greater the number of connections that are loaded onto one trunk by the routing algorithm. The higher the number of data with DFM and voice connections, the higher the probability of Tx TS Pkt Drp errors.

3. In order for switch software to allocate the correct amount of bandwidth required on the BTM trunk, the configured utilization value must reflect actual activity on the connection.

   If the configured value does not reflect the actual activity on the channel, the switch software allocates an incorrect amount of minimum bandwidth. This discrepancy occurs when DFM does not work due to constant changes in data patterns from the end application. An incorrect utilization configuration for one connection usually does not cause any problems. However, packet drops could result if the configured utilization of many DFM data connections is lower than actual utilization.

   1. Issue the SuperUser-level commands cnftrkstats, dsptrkstatcnf, and dsptrkstathist to collect and view target trunk statistics. The statistics require significant network process module (NPM) processing time. Enable them only for troubleshooting activities.

   2. Use Cisco WAN Manager statistics to evaluate long term trunk use.

4. Issue the cnftrkparm command to check all trunk parameters.

   1. Compare the problem trunk settings to the default values or to other trunks that have similar traffic without Tx TS Pkt Drp errors. An important parameter is the TS setting of Transmit Queue Depth, which you can display with the cnftrkparm command.

   2. Consider network specific requirements before you change the queue depth because changes affect all connections that share the TS queue on the BTM trunk.

If the problem persists after you perform the troubleshooting steps, contact the Cisco Systems Technical Support at (800) 553-24HR, (408) 526-7209, the Cisco Technical Support Website, or send an e-mail to tac@cisco.com.

Related Information

• More BTM Trunk Alarm Types
• How to Distinguish Between Different IGX NTM Models
• International Telephony Union (ITU) Recommendation G.704
• Cisco WAN Switching Solutions - Cisco Documentation
• Guide to New Names and Colors for WAN Switching Products
• Downloads - WAN Switching Software
• Technical Support - Cisco Systems