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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 20 Configuring QoS
QoS Configuration Examples
For the Catalyst 3500 XL and 2900 XL switches, CoS configures each egress port with a normal-priority
transmit queue and a high-priority transmit queue, depending on the frame tag or the port information.
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 are placed in the normal-priority transmit
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
Figure 20-10
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
Step 1
configure terminal
Enter global configuration mode.
Step 2
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.
Step 3
class-map videoclass
Create a class map called videoclass, and enter class-map
configuration mode.
Step 4
match access-group 1
Define the match criterion by matching the traffic specified by
access list 1.
Step 5
exit
Return to global configuration mode.
Step 6
policy-map videopolicy
Create a policy map called videopolicy, and enter policy-map
configuration mode.
Step 7
class videoclass
Specify the class on which to act, and enter policy-map class
configuration mode.
Step 8
set ip dscp 56
For traffic matching ACL 1, set the DSCP of incoming packets
to 56.
Step 9
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.
Step 10
exit
Return to policy-map configuration mode.
Step 11
exit
Return to global configuration mode.
Step 12
interface gigabitethernet0/1
Enter interface configuration mode, and specify the ingress
interface.
Step 13
service-policy input videopolicy
Apply the policy to the ingress interface.
Step 14
exit
Return to global configuration mode.
Step 15
interface gigabitethernet0/2
Enter interface configuration mode, and specify the egress
interface (to configure the queues).
Step 16
priority-queue out
Enable the expedite queue.
