Queues And Wtd Thresholds - Cisco Catalyst 2960 series Configuration Manual

Information About QoS
are reserved for it by setting a maximum threshold. The switch can allocate the needed buffers from the
common pool if the common pool is not empty.
The switch supports 4 egress queues by default, although there is an option to enable a total of 8 egress
Note
queues. Use the mls qos srr-queue output queues 8 global configuration command to enable all 8 egress
queues. Once 8 egress queues are enabled, you are able to configure thresholds and buffers for all 8 queues.
The 8 egress queue configuration is only supported on a standalone switch.

Queues and WTD Thresholds

You can assign each packet that flows through the switch to a queue and to a threshold.
Specifically, you map DSCP or CoS values to an egress queue and map DSCP or CoS values to a threshold
ID. You use the mls qos srr-queue output dscp-map queue queue-id {dscp1...dscp8 | threshold threshold-id
dscp1...dscp8} or the mls qos srr-queue output cos-map queue queue-id {cos1...cos8 | threshold threshold-id
cos1...cos8} global configuration command. You can display the DSCP output queue threshold map and the
CoS output queue threshold map by using the show mls qos maps privileged EXEC command.
The queues use WTD to support distinct drop percentages for different traffic classes. Each queue has three
drop thresholds: two configurable (explicit) WTD thresholds and one nonconfigurable (implicit) threshold
preset to the queue-full state. You assign the two WTD threshold percentages for threshold ID 1 and ID 2.
The drop threshold for threshold ID 3 is preset to the queue-full state, and you cannot modify it. You map a
port to queue-set by using the queue-set qset-id interface configuration command. Modify the queue-set
configuration to change the WTD threshold percentages.
The switch supports 4 egress queues by default, although there is an option to enable a total of 8 egress
Note
queues. Use the mls qos srr-queue output queues 8 global configuration command to enable all 8 egress
queues. Once 8 egress queues are enabled, you are able to configure thresholds and buffers for all 8 queues.
The 8 egress queue configuration is only supported on a standalone switch.
Related Topics
Weighted Tail Drop, on page 554
Shaped or Shared Mode
SRR services each queue-set in shared or shaped mode. You map a port to a queue-set by using the queue-set
qset-id interface configuration command. You assign shared or shaped weights to the port by using the
srr-queue bandwidth share weight1 weight2 weight3 weight4 or the srr-queue bandwidth shape weight1
weight2 weight3 weight4 interface configuration command.
The buffer allocation together with the SRR weight ratios control how much data can be buffered and sent
before packets are dropped. The weight ratio is the ratio of the frequency in which the SRR scheduler sends
packets from each queue.
All four queues participate in the SRR unless the expedite queue is enabled, in which case the first bandwidth
weight is ignored and is not used in the ratio calculation. The expedite queue is a priority queue, and it is
serviced until empty before the other queues are serviced. You enable the expedite queue by using the
priority-queue out interface configuration command.
You can combine the commands described in this section to prioritize traffic by placing packets with particular
DSCPs or CoSs into certain queues, by allocating a large queue size or by servicing the queue more frequently,
and by adjusting queue thresholds so that packets with lower priorities are dropped.
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