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Cisco IE 3010 Switch Software Configuration Guide
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Chapter 33 Configuring QoS
Understanding QoS
SRR services the priority queue for its configured weight as specified by the bandwidth keyword in the
mls qos srr-queue input priority-queue queue-id bandwidth weight global configuration command.
Then, SRR shares the remaining bandwidth with both ingress queues and services them as specified by
the weights configured with the mls qos srr-queue input bandwidth weight1 weight2 global
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. For
configuration information, see the “Configuring Ingress Queue Characteristics” section on page 33-65.
Queueing and Scheduling on Egress Queues
Figure 33-9 shows the queueing and scheduling flowchart for egress ports.
Note If the expedite queue is enabled, SRR services it until it is empty before servicing the other three queues.
Figure 33-9 Queueing and Scheduling Flowchart for Egress Ports
Each port supports four egress queues, one of which (queue 1) can be the egress expedite queue.These
queues are configured by a queue-set. All traffic leaving an egress port flows through one of these four
queues and is subjected to a threshold based on the QoS label assigned to the packet.
Figure 33-10 shows the egress queue buffer. The buffer space is divided between the common pool and
the reserved pool. The switch uses a buffer allocation scheme to reserve a minimum amount of buffers
for each egress queue, to prevent any queue or port from consuming all the buffers and depriving o ther
queues, and to control whether to grant buffer space to a requesting queue. The switch detects whether
the target queue has not consumed more buffers than its reserved amount (under-limit), whether it has
consumed all of its maximum buffers (over limit), and whether the commo n pool is empty (no free
buffers) or not empty (free buffers). If the queue is not over-limit, the switch can allocate buffer space
from the reserved pool or from the common pool (if it is n ot empty). If there are no free buffers in the
common pool or if the queue is over-limit, the switch drops the frame.
Figure 33-10 Egress Queue Buffer Allocation
Port 1 queue 1
Port 1 queue 2
Port 1 queue 3
Port 1 queue 4
Port 2 queue 1
Port 2 queue 2
Common pool
Reserved pool
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