Congestion Management and Congestion Avoidance 319
As noted earlier in the chapter, when you configure QoS on the Catalyst 6500, the ASICs
are being programmed directly. Therefore, when ports and interfaces share the same ASIC,
they inherit or share QoS configuration parameters. Example 8-36 demonstrates this
behavior. Also the strict-priority queue is depicted directly above. By default, frames
marked with CoS 5 are mapped to the strict-priority queue. To map other CoS values to the
strict-priority queue, utilize priority-queue cos-map 1 {CoS} for Native mode and set qos
map {port type} tx {queue#} {thr#} cos {CoS} when running Hybrid. In Hybrid, queues
are numbered starting with the lowest-priority queue and ending with the highest-priority
queue. As an example, there are three available queues for 1p2q2t port types. The first
queue represents the lowest standard priority queue available and offers the least amount of
insurance for network traffic. However, queue three in this case is the strict-priority queue,
and as a result offers a low drop probability and minimal delay during periods of
congestion. After the CoS values have been mapped to a corresponding queue and
threshold, it is important to analyze the scheduling between the different queues. This
analysis ensures high-priority and mission-critical traffic is receiving sufficient bandwidth
during periods of congestion.
Configuring Output Queue Scheduling WRR Weighting Factors
Covered in Chapter 2, WRR is a scheduling mechanism used to determine the frequency
with which certain queues are serviced. Unlike round-robin, which offers each participating
queue equal access to the allocated bandwidth, WRR enables the administrator to assign a
weighting factor allowing queues to utilize a greater share of the bandwidth. If a queue has
a higher weight, it is permitted to transmit more frames onto the network. Hence, it is
serviced more frequently than other queues. With the exception of the strict-priority queue,
all port transmit queues participate in the WRR scheduling process. Figure 8-12 shows the
WRR operation among multiple queues.
Example 8-36 Mapping CoS Value to Transmit Queue and Threshold in Native Mode
native(config-if)# wrr-queue cos-map 2 1 3
cos-map configured on: Gi2/1 Gi2/2 Gi2/3 Gi2/4 Gi2/5 Gi2/6 Gi2/7 Gi2/8
native(config-if)# wrr-queue cos-map 2 2 6
cos-map configured on: Gi2/1 Gi2/2 Gi2/3 Gi2/4 Gi2/5 Gi2/6 Gi2/7 Gi2/8
native# show queueing interface gigabitEthernet 2/1 | begin queue thresh
queue thresh cos-map
---------------------------------------
1 1 0 1
1 2 2
2 1 3 4
2 2 6 7
3 1 5
(text omitted)