WRED Configuration Examples
This section explains how to configure different treatment of colored packets, different drop behavior for each queue, RED and dynamic queue thresholds, and average queue lengths for WRED.
Configuring Different Treatment of Colored Packets for WRED
Figure 5 shows a WRED drop profile that yields progressively more aggressive drop treatment for each color. Exceeded traffic is dropped over a wider range and with greater maximum drop probability than conformed or committed traffic. Conformed traffic is dropped over a wider range and with greater maximum drop probability than committed traffic.
The commands to configure this example are:
host1(config)#drop-profile wredColored
host1(config-drop-profile)#committed-threshold percent 30 90 3
host1(config-drop-profile)#conformed-threshold percent 25 90 5
host1(config-drop-profile)#exceeded-threshold percent 20 90 10
Defining Different Drop Behavior for Each Traffic Class
You can define different dropping behaviors for each traffic class in the router. By doing so, you can assign less aggressive drop profiles to higher-priority queues and more aggressive drop profiles to lower-priority queues. Figure 6 shows an example that classifies packets into one of four traffic classes. Each traffic class has a different queueing behavior, drop treatment, and scheduler treatment.
Configuring WRED and Dynamic Queue Thresholds
RED typically operates on fixed-size queues, and you can configure the router to use fixed-size queues. However, by default, the router employs dynamic queue thresholds to provide a good balance between sharing the egress buffer memory between queues and protecting an individual queue's claim on its fair share of the egress memory. Fixed-size queues become problematic as the number of configured queues scales into the thousands, because allocating disjointed partitions of buffer memory to each queue means the allocations become quite small, and most likely not all queues are simultaneously active.
In general, you use queues as follows:
- Fixed-size queues on core routers and core-facing interfaces where the number of queues is relatively small (tens or hundreds, but not thousands).
- Dynamic queues on edge-facing interfaces where the number of queues is relatively large (thousands).
As shown in Figure 7, queue lengths extend to oversubscribe memory when aggregate memory utilization is low, and contract to strictly partition memory when memory utilization is high. Dynamic thresholding enforces fairness when free buffers are scarce and promotes sharing when buffers are plentiful. Dynamic queue thresholds are discussed in Queuing and Buffer Management Overview. Figure 7 illustrates WRED behavior with dynamic queue thresholding.
To configure WRED to run on queues whose limits dynamically expand and contract, use the percent keyword when you configure thresholds in a drop profile. For example:
host1(config)#drop-profile internetDropProfile
host1(config-drop-profile)#average-length-exponent 9
host1(config-drop-profile)#committed-threshold percent 30 90 4
host1(config-drop-profile)#conformed-threshold percent 25 90 5
host1(config-drop-profile)#exceeded-threshold percent 20 90 6
