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JunosE 11.3.x Quality of Service Configuration Guide
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RED and WRED Overview
26
By default, tail dropping occurs when the length of a queue exceeds a threshold. Drop
profiles allow you to employ active queue management by specifying RED and WRED
parameters to be applied to an egress queue.
Congestion of an egress queue occurs when the rate of traffic destined for the queue
exceeds the rate of traffic draining from the queue; the queue fills to its limit, and any
further traffic destined to it must be discarded until there is room in the queue. RED and
WRED monitor average queue length over time to detect incipient congestion.
You can combine drop profiles and queue profiles within a queue rule of a QoS profile
to specify up to 256 unique queuing behaviors within the router. You can then associate
these queuing behaviors in any combination with any of the egress queues.
Queuing and Buffer Management Overview on page 17
The scheduler maintains an average queue length for each queue configured for RED.
When a packet is enqueued, the current queue length is weighted into the average queue
length based on the average-length exponent in the drop profile.
Small exponent values weight the current queue length heavily, so the average queue
length is more responsive to transient bursts.
Large exponent values weight the current queue length lightly, so the average queue
length is less responsive to bursts.
When the average queue length exceeds the minimum threshold, RED begins randomly
dropping packets. While the average queue length increases toward the maximum
threshold, RED drops packets with increasing frequency, up to the maximum drop
probability. When the average queue length exceeds the maximum drop threshold, all
packets are dropped. Figure 2 on page 26 shows this behavior.
Figure 2: Packets Dropped as Queue Length Increases
Copyright © 2010, Juniper Networks, Inc.
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