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Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 26 Configuring QoS
QoS Configuration Examples
QoS Configuration for the Existing Wiring Closet
The existing wiring closet in Figure 26-4 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 classify untagged (native) Ethernet
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 XL and 3548
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 Catalyst 3550-12T switch 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 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 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 Figure 26-4 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
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
ACL 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 46 For traffic matching ACL 1, set the DSCP of incoming packets
to 46.