Cisco Catalyst 3750 Software Configuration Manual page 498

Understanding QoS
Each port supports four egress queues. These queues are assigned to a queue-set. All traffic exiting the
switch on a standard port flows through one of these four queues and is subjected to a threshold based
on the QoS label assigned to the packet. Traffic destined for an ES port passes through the queue-set
before reaching the hierarchical queues. If congestion occurs in the hierarchical queues that backs up to
the queue-sets, the queue-set configuration controls how traffic is dropped.
Figure 26-9
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
other queues, and to control whether to grant buffer space to a requesting queue. The switch detects
whether or not the target queue has consumed more buffers than its reserved amount (under-limit),
whether it has consumed all of its maximum buffers (over limit), and whether the common 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 not empty). If there are no free buffers in
the common pool or if the queue is over-limit, the switch drops the frame.
Figure 26-9 Egress Queue-Set Buffer Allocation
Buffer and Memory Allocation
You guarantee the availability of buffers, set drop thresholds, and configure the maximum memory
allocation for a queue-set by using the mls qos queue-set output qset-id threshold queue-id
drop-threshold1 drop-threshold2 reserved-threshold maximum-threshold global configuration command.
Each threshold value is a percentage of the queue's allocated memory, which you specify by using the
mls qos queue-set output qset-id buffers allocation1 ... allocation4 global configuration command.
The sum of all the allocated buffers represents the reserved pool, and the remaining buffers are part of
the common pool.
Through buffer allocation, you can ensure that high-priority traffic is buffered. For example, if the buffer
space is 400, you can allocate 70 percent of it to queue 1 and 10 percent to queues 2 through 4. Queue
1 then has 280 buffers allocated to it, and queues 2 through 4 each have 40 buffers allocated to them.
You can guarantee that the allocated buffers are reserved for a specific queue in a queue-set. For
example, if there are 100 buffers for a queue, you can reserve 50 percent (50 buffers). The switch returns
the remaining 50 buffers to the common pool. You also can enable a queue in the full condition to obtain
more buffers than 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.
Catalyst 3750 Metro Switch Software Configuration Guide
26-18
shows the egress queue-set buffer. The buffer space is divided between the common pool
Chapter 26
Common pool
Reserved pool
Configuring QoS
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