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Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapt er 26 Conf i gurin g QoS
QoS Configuration Examp les

QoS Configuration for the Existing Wiring Closet

The existing wiring closet in Figure 26-4 cons ist s of existing Ca talyst 2900 XL and 3500 XL switc hes .
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
pref eren ce to hi gh er-pri ori ty tra ffic.
Recall tha t on the Cata lyst 2 900 a nd 3500 XL s witc hes, you can c lassif y u nta gged (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 fr om t he heade r frame is use d . On the Ca talyst 3524-PWR XL and 3548
XL switches, you can override this priority with the default value by using the switchport priority
default override inter face configurati on comman d. For Catalyst 295 0 and Cata lyst 2900 XL switc hes
and ot her 3500 X L m ode ls t hat do not h ave the override fea ture , t he Ca taly st 3550- 12T swi tch at the
distribution l ayer can override the 8 02.1P C oS value by using the mls qos c os override interface
configurati on comm a nd.
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
inform at ion. Fram e s i n t he no rmal -p rior ity qu eu e ar e fo rwarde d only a fte r fra me s in t he h i gh- prior it y
queue ar e forwarde d. Frames th at have 802. 1P CoS valu es of 0 to 3 are placed in the norm al-prior ity
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 intelligen t wiring closet in Figure 26-4 is composed of Catalyst 2950 switches. One of the switches
is conn ect ed t o a vid eo server, which h as an I P a ddress of 17 2.20. 10. 16.
The ob ject of this example is to pri o ritize the video traffic over al l ot her traffic. To do so, a DS CP of 46
is assigned to the video traffic. This traffic is stored in queue 4, which is serviced more frequently than
the oth er qu eu es.
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 g lob al c onfigura tion m ode .
Step 2 access-l ist 1 permi t 172.20.10 .16 Define an IP stan dard ACL, and pe rmit tra ffic from the video
server at 172 .20.10.1 6.
Step 3 class-map vi deoclass Crea te a clas s map ca lled videoclass, and enter cl ass-m ap
configuratio n mode .
Step 4 match access-group 1 Define the match criterion by matching the traffic specified by
AC L 1 .
Step 5 exit Return to glob al co nfiguration mo de.
Step 6 policy-map videopolicy Creat e a policy map calle d videopolicy, and enter policy-map
configuratio n mode .
Step 7 class videoclass Spec ify th e clas s on whi ch to ac t, and en ter po licy-m ap clas s
configuratio n mode .
Step 8 set ip dscp 46 For traffic matching ACL 1, set the DSCP of incoming packets
to 46.