Cisco Catalyst 3550 series Software Configuration Manual page 602

Understanding QoS
Each minimum-reserve level is configured with a buffer size. As shown in the figure, queue 4 of Fast
Ethernet port 0/1 has a buffer size of 70 packets, queue 4 of Fast Ethernet port 0/2 has a buffer size of
80 packets, queue 4 of Fast Ethernet port 0/3 has a buffer size of 40 packets, and Fast Ethernet port 0/4
has a buffer size of 80 packets. You configure the buffer size by using the mls qos min-reserve global
configuration command.
Figure 29-7 10/100 Ethernet Port Queue Assignment, Minimum-Reserve Levels, and Buffer Size
Fast Ethernet
Port Number
* MRL = Minimum-reserve level
The available bandwidth of the egress link is divided among the queues. You configure the queues to be
serviced according to the ratio of WRR weights by using the wrr-queue bandwidth interface
configuration command. The ratio represents the importance (weight) of a queue relative to the other
queues. WRR scheduling prevents low-priority queues from being completely neglected during periods
of high-priority traffic by sending some packets from each queue in turn. The number of packets sent
corresponds to the relative importance of the queue. For example, if one queue has a weight of 3 and
another has a weight of 4, three packets are sent from the first queue for every four that are sent from the
second queue. By using this scheduling, low-priority queues can send packets even though the
high-priority queues are not empty. Queues are selected by the CoS value that is mapped to an egress
queue (CoS-to-egress-queue map) through the wrr-queue cos-map interface configuration command.
All four queues participate in the WRR unless the egress expedite queue is enabled, in which case, the
fourth bandwidth weight is ignored and not used in the ratio calculation. The expedite queue is a priority
queue, and it is serviced until empty before the other queues are serviced. You enable the expedite queue
by using the priority-queue out interface configuration command.
You can combine the commands described in this section to prioritize traffic by placing packets with
particular DSCPs into certain queues, allocate a larger minimum-reserve buffer size, and service a particular
queue more frequently. For configuration information, see the
Ethernet Ports" section on page
Catalyst 3550 Multilayer Switch Software Configuration Guide
29-16
Q1 Q2 Q3
MRL* MRL MRL
0/1 1 3
0/2 2 4
0/3 1 2
0/4 5 6
•
•
•
29-64.
MRL
Q4
1
MRL
2
5 7
3
6 8
4
3 4
5
7 8
6
7
8
Chapter 29
Buffer size
10
20
30
40
50
60
70
80
"Configuring Egress Queues on 10/100
Configuring QoS
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