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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter36 Co nf iguring QoS
Understanding QoS
These queues are assigned to a queue-set. All traffic exiting the switch flows through one of these four
queues and is subjected to a threshold based on the QoS label assigned to the packet.
Figure 36-10 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 the r
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 p ool is e mpty (n o fre e
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 emp t y). I f ther e a re no f ree buffers in t he
common pool or if the queue is over-limit, the switch drops the frame.
Figure36-10 Egress Queue 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-thresh old 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 traf f ic is buf fered. F or ex ample, if the b uffe r
space is 400, you can allocate 70 percent of it to queue 1 and 10 percent to queue s 2 through 4. Queue 1
then has 280 buffers allocated to it, and queues 2 through 4 each have 40 buffers allocated to the m.
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 swit ch returns
the remaining 50 buffers to the common pool. You also can enable a queue in the full conditio n to obtain
more buffers than are reserved for it by setting a maxim um threshold. The switch can allocate th e needed
buffers from the common pool if the common pool is not empty.
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 poo
l
Reserved poo
l
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