Chapter 13 - Tuning Your Switch Performance (Layer 2 & Layer 3); Overview; How Queues Work - Lucent Technologies Cajun P220 Operation Manual

13
Tuning Your Switch
Performance (Layer 2 & Layer 3)

Overview

This chapter and its procedures are common to both Layer 2 and Layer 3 configuration.
Buffer management features help you to optimize traffic throughput through the switch
fabric.
Included in this chapter:

How Queues Work

Managing Buffers and Queues
How Queues Work
Frames are buffered in the I/O modules, before and after traversing the switch. Each
queue can hold 256K bytes. (Architecturally they can support up to 1 MB each).
Each buffer is divided into two queues, one for High-priority Traffic and one for
Normal-priority Traffic. The factory default is for the high-priority queue uses 20%
(51K) of the buffer. The normal-priority queue uses the remaining 80% (205K). These
values can be modified using either the Web Agent or SNMP.
Note: When you change these values, you must reboot the switch before they can
take effect.
Less buffer memory gets assigned to the high-priority queue because the high-priority
queue gets serviced more frequently than the normal-priority queue. Since a frame
spends less time on the high-priority queue, less buffer space is required for the queue.
The Service Ratio can be chosen to match traffic patterns and performance requirements
using a weighted round robin scheduling algorithm. The available service ratios of the
algorithm are defined in "Managing Buffers and Queues". The factory default service
ratio for fabric ports is 999/1. The factory default service ratio for physical ports is 1023
to 1. If there is traffic to be serviced from both the high- and normal-priority queues, 999
packets of high-priority traffic will be processed for each normal-priority packet.
Cajun P550/P220 Switch Operation Guide 13-1