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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter20 Co nf iguring QoS
QoS Configuration Examples
For the Catalyst 3500 XL and 2900 XL switches, CoS config ures each egr ess po rt with a norm al-priori ty
transmit queue and a high-priority transmit queue, depend in g on t he fr am e t ag o r the por t i nf ormat ion .
Frames in the normal-priority queue are forwarded only after frames in the high-priority queue are
forwarded. Frames that have 802.1P CoS values of 0 to 3 a re p lac ed i n the no rmal -p rior ity t ransm i t
queue whereas frames with CoS values of 4 to 7 are placed in the expedite (high-priority) queue.
QoS Configuration for the Intelligent Wiring Closet
The intelligent wiring closet in Figure20-10 is composed of Catalyst 3550 multilayer switches. One of
the switches is connected to a video server, which has an IP address of 172.20. 10 .16 .
The object of this example is to prioritize the video traffic over all other traffic. To do so, a DSCP of 56
is assigned to the video traffic. This traffic is stored in the expedite queue (queue 4), which is serviced
until empty before the other queues are serviced. The appropriate CoS value selects queue 4 in the
CoS-to-egress-queue map.
Beginning in privileged EXEC mode, follow these steps to configure the switch to prioritize video
packets over all other traffic:
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 access-list 1 permit 172.20.10.16 Define an IP standard ACL, and permit traffic from the video
server at 172.20.10.16.
Step3 class-map videoclass Create a class map called videoclass, and enter class-map
configuration mode.
Step4 match access-group 1 Define the match criterion by matching the traffic specified by
access list 1.
Step5 exit Return to global configuration mode.
Step6 policy-map videopolicy Create a policy map called videopolicy, and enter policy-map
configuration mode.
Step7 class videoclass Specify the class on which to act, and enter policy-map class
configuration mode.
Step8 set ip dscp 56 For traffic matching ACL 1, set the DSCP of incoming packets
to 56.
Step9 police 5000000 2000000 exceed-action drop Define a policer for the classified video traffic to drop traffic that
exceeds 5-Mbps average traffic rate with a 2-MB burst size.
Step10 exit Return to policy-map configuration mode.
Step11 exit Return to global configuration mode.
Step12 interface gigabitethernet0/1 Enter interface configuration mode, and specify the ingress
interface.
Step13 service-policy input videopolicy Apply the policy to the ingress interface.
Step14 exit Return to global configuration mode.
Step15 interface gigabitethernet0/2 Enter interface configuration mode, and specify the egress
interface (to configure the queues).
Step16 priority-queue out Enable the expedite queue.