BigIron RX Series Configuration Guide 48153-1001986-01Configuring packet drop priority using WRED 18Using WRED with rate limitingWhen rate limiting is configured on a device, it directs the switch to drop traffic indiscriminatelywhen the configured average-rate and maximum-burst thresholds are exceeded. If rate limiting isconfigured with WRED, the traffic that exceeds these thresholds can be subjected to the WREDalgorithm which drops packets selectively by priority.In this configuration, packets that exceed the thresholds established by the rate limitingconfiguration are marked as either exceeding the average-rate or maximum-burst threshold. Thismarking is then used to select a WRED configuration that determines which packets to drop.Configuring packet drop priority using WREDFor a description of WRED, refer to “Determining packet drop priority using WRED” on page 479.This section describes how to configure the parameters described in that section to enable the useof WRED on a device.To configure WRED, you must configure the following parameters:• “Enabling WRED”• “Setting the averaging-weight (Wq) parameter”• “Configuring the drop precedence parameters”Enabling WREDWRED must be enabled on any forwarding queue that you want it to operate on. To enable WREDfor the forwarding queue 3, enter the following command.BigIron RX(config)#qos queue-type 3 wred enableSyntax: [no] qos queue-type wred enableThe variable is the number of the forwarding queue that you want to enableWRED for. There are four forwarding queues on device. They are numbered 0 to 3 with zero as thelowest priority queue and three the highest.Setting the averaging-weight (Wq) parameterThe Wq parameter is configured as the averaging-weight parameter. In this implementation, youcan set one of 13 (1 - 13) possible values. These values represent a Wq value as described inTable 94pkt-size (avg-q-size - min-avg-q size)Pdrop = ----------------- * Pmax * -----------------------------------------pkt-size-max (max-avg-q-size - min-avg-q size)TABLE 94 Possible Wq valuesAveraging weightsettingWq value as a percentage1 50%2 25%
