Cisco WAN Switching Modules

Document ID: 10861

Contents

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

Introduction

This document discusses Broadband Trunk Module (BTM) packet drop errors and provides steps to troubleshoot these errors.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

The information in this document applies to the Cisco IGX™ BTM with E1, E2, E3, and T3 backcards.

Conventions

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

Error Definition

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

Rx BData A Pkt Drp can have these causes:

1. Issue the SuperUser-level command dsptrkutl, to check for high trunk utilization. Packet drops can occur when trunk utilization reaches 85 percent. Frame Relay (FR) traffic is very bursty, which causes short-term variations in the load on BData A trunk queues. While BData A queues are relatively large by default, they can not cope with all possible statistical events, such as correlated bursts from many connections. Occasional BData A Packet Drops generally pose no problem.

   If packet drops from the BData A queue continue, verify the utilization assumptions for the FR connections. The FR connection settings of minimum information rate (MIR), percent utilization (%Util), and credit maximum (Cmax) are particularly important, if packet drops continue.

   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 is the case, the most likely symptom is end-system retransmission or timeouts.

2. Misconfigured trunk parameters, as indicated by the SuperUser-level command cnftrkparm.

Error Example

The likely location of equipment errors is highlighted in yellow in this diagram:

• NT—Network Termination

• MUX—The Multiplexer in the Telco line path.

• Rx—Receive

• Tx—Transmit

Troubleshooting

Follow these steps to troubleshoot the error:

1. Issue the SuperUser-level dsptrkutl trunk_number command, to check the 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 find current trunk utilization. If the Peak Interval Utilization field is higher than 85 percent, then queue overflows are causing packet drops.

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

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

   5. With the aforementioned commands, verify the trunk errors and the utilization at the distant end.

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

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

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

   9. Issue these SuperUser-level commands to display utilization for each connection routed across the problem trunk:

      Connection	Command
      voice connection	dsputl
      data connection	dspdutl
      ATM or FR connection	dspchstats

2. For FR connections, the parameters that affect trunk bandwidth allocation for a connection are MIR and %Util.

   1. Issue the dspcon command to verify the MIR and %Util settings.

   2. To change the MIR or %Util, issue the cnfcon command.

      The lower the %Util for an FR connection, the greater the number of FR connections that the routing algorithm can load onto one trunk. The higher the number of FR connections, the higher the probability of RX BData A Pkt Drp errors. For switch software to allocate the correct amount of bandwidth that is required on the BTM trunk, the configured %Util value must reflect actual activity on the connection. Packet drops result when the configured utilization of many FR connections is lower than actual use.

   3. Issue the cnftrkstats, dsptrkstatcnf, and dsptrkstathist commands to collect and view target trunk statistics.

      Note: These statistics require significant NPM processing time and should only be enabled for troubleshooting activities.

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

3. Issue the dspcon command to check the Cmax setting on the connection.

   Cmax determines the size of initial bursts that are allowed into the network at port speed. A large Cmax setting increases the probability of RX BData A Pkt Drp errors. The default value is ten and should not be changed without investigation.

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 packet drops. An important parameter for data connections is the Receive Queue Depth BDataA field of cnftrkparm.

   2. Consider network-specific requirements before you change the queue depth, because of the impact that changes will have on all BData A connections on this trunk.

If the problem persists after you perform the troubleshooting steps, contact Cisco Systems Technical Support:

• Phone: (800) 553-24HR or (408) 526-7209

• Website: Technical Support - Cisco Systems

• E-mail: tac@cisco.com

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Related Information

• Cisco WAN Switching Solutions - Cisco Documentation
• Guide to New Names and Colors for WAN Switching Products
• Software Center - WAN Switching Software
• Technical Support - Cisco Systems