Juniper JUNOSE 11.1.X - QUALITY OF SERVICE CONFIGURATION GUIDE 3-21-2010, Configuration Manual

Page: 58 / 398

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.
Related Topics
■
Queuing and Buffer Management Overview on page 17
RED and WRED Overview
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 27 shows this behavior.
26 ■
RED and WRED Overview
JUNOSe 11.1.x Quality of Service Configuration Guide