Cisco Systems 3750E manual 36-19

Chapter 36 Configuring QoS

Understanding QoS

Figure 36-12 Queueing and Scheduling Flowchart for Egress Ports on Catalyst 3560-E Switches

Start
Receive packet from
the internal ring.
Read QoS label
(DSCP or CoS value).
Determine egress queue
number and threshold
based on the label.
Are thresholds	Yes
being exceeded?
No

Queue the packet. Service

the queue according to

the SRR weights.

Rewrite DSCP and/or

CoS value as appropriate.

Send the packet

out the port.

Done

Drop packet.

90565

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 36-13shows 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 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-E and 3560-E Switch Software Configuration Guide
OL-9775-02		36-19