Cisco Systems 3750E manual Buffer and Memory Allocation, 36-20

Chapter 36 Configuring QoS

Understanding QoS

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 36-13 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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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-idthreshold queue-iddrop-threshold1drop-threshold2reserved-thresholdmaximum-thresholdglobal 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-idbuffers 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-E and 3560-E Switch Software Configuration Guide
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