Cisco WAN Switching Modules

Document ID: 10864

Contents

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

Introduction

This document provides information you can use to troubleshoot broadband trunk module (BTM) Rx NTS 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

This error applies 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.

RxNon-TS Pkt Drp causes 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 data connections with data frame multiplexing (DFM), trunk bandwidth is assigned to each connection and is based on the configuration connection. DFM provides data compression with the use of a repetitive, pattern-suppressing algorithm.

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

  Because actual fill grade can vary quickly for the nontimestamped (Non-TS) queue, and because the rate of FastPackets per connection depends on the traffic patterns of the end system, packet drops on trunk queues typically do not follow predictable patterns. However, more drops occur during peak hours than during 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 by the SuperUser-level command cnftrkparm.

Error Example

The likely location of equipment errors is highlighted in yellow.

Troubleshoot

Complete these steps in order to troubleshoot the Rx NTS Pkt Drp error.

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

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

   2. When dsptrkerrs shows dropped packets, issue the dsptrkutl command to find current trunk utilization. If the Peak Interval Utilization field is higher than 85, queue overflows cause the packet drops.

   3. 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. Verify trunk errors and utilization at the distant end by using the dsptrkerrs and dsptrkutl commands.

   4. For immediate relief, route high-speed data and non-VAD voice connections over alternate trunks. Issue the SuperUser-level dsptrkcons command to identify the number of connections that route over the problem trunk.

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

   6. Issue these SuperUser-level commands in order to display utilization for each connection that routes across the problem trunk.

      • dsputl - voice connection.

      • dspdutl - data connection.

      • dspchstats - ATM or Frame Relay connection.

2. In order to resolve the problem, evaluate the network design of connection parameters and correct as necessary. For non-VAD voice and high speed data connections, utilization is the parameter that affects trunk bandwidth allocation.

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

   2. In order to change utilization, issue the cnfchutl command. The lower the configured utilization for a voice connection, the greater the number of voice connections the routing algorithm can load onto a trunk. The higher the number of connections, the higher the probability of Rx Non-TS Pkt Drp errors.

3. 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 because data patterns constantly change from the end application. An incorrect utilization configuration for one connection usually does not cause any problems. However, packet drops can result when the configured utilization of many DFM data connections is lower than actual utilization.

   1. Use 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 in order to check all trunk parameters.

   1. Compare the problem trunk settings to the default values or to other trunks that have similar traffic without packet drops. Important parameters for non-VAD voice and high-speed data connections are the Receive Queue Depth settings for the Non TS field of cnftrkparm.

   2. Consider network specific requirements before you change the queue depth because changes affect all voice connections on this trunk.

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

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