Cisco Catalyst 3750-X Software Configuration Manual page 900

Understanding QoS
Figure 39-12
Receive packet from
(DSCP or CoS value).
Determine egress queue
number and threshold
based on the label.
Queue the packet. Service
the queue according to
Rewrite DSCP and/or
Each port supports four egress queues, one of which (queue 1) can be the egress expedite queue. 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 39-13
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
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 common pool is empty (no free
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
39-20
Queueing and Scheduling Flowchart for Egress Ports on Catalyst 3560-X Switches
Start
the internal ring.
Read QoS label
Yes
Are thresholds
being exceeded?
No
the SRR weights.
CoS value as
appropriate.
Send the packet
out the port.
Done
shows the egress queue buffer. The buffer space is divided between the common pool and
Drop packet.
Chapter 39 Configuring QoS
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