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Catalyst2950 Desktop Switch Software Configuration Guide
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Chapter25 Co nf iguring QoS
QoS Configuration Examples
QoS Configuration for the Existing Wiring Closet
The existing wiring closet in Figure25- 4 consists of existing Catalyst 2900 XL and 3500 XL switches.
These switches are running IOS release 12.0(5)XP or later, which supports the QoS-based IEEE 802.1P
CoS values. QoS classifies frames by assigning priority-indexed CoS values to them and gives
preference to higher-priority traffic.
Recall that on the Catalyst 2900 and 3500 XL switches, you can c lassif y u nta gge d (na tive) Ether net
frames at the ingress ports by setting a default CoS priority (switchport priority default
default-priority-id interface configuration command) for each port. For IEEE 802.1Q frames with tag
information, the priority value from the header frame is used. On the Catalyst 3524-PWR X L and3548
XL switches, you can override this priority with the default value by using the switchport priority
default override interface configuration command. For Catalyst 2950 and Catalyst 2900 XL switches
and other 3500 XL models that do not have the override feature, the Ca taly st 35 50-12T swi tch at the
distribution layer can override the 802.1P CoS value by using the mls qos cos override interface
configuration command.
For the Catalyst 2900 and 3500 XL switches, CoS configures each transmit port (the egress p ort) with a
normal-priority transmit queue and a high-priority transmit queue, dependin g on the frame tag or the port
information. Frames in the normal-priority queue ar e fo rwarde d only a fte r fra me s in t he h i gh- prior it y
queue are forwarded. Frames that have 802.1P CoS values of 0 to 3 are placed in the normal-priority
transmit queue while 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 Figure25- 4 is composed of Catalyst 2950 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 46
is assigned to the video traffic. This traffic is stored in queue 4, which is serviced more frequently than
the other queues.
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
ACL 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 46 For traffic matching ACL 1, set the DSCP of incoming packets to
46.