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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapt er 26 Conf i gurin g QoS
Unders tanding QoS
•Video wizard—Gives traffic th at origin ates from speci fied vi deo servers a hi gher prio rity than the
priority of data traffic. The wizard assumes that the video servers are connected to a single device
in the clus ter. Refer to the vi deo wizar d online hel p for proce dures abou t using this wizard.
This chapter consists of these sections:
•Understa ndin g QoS, pag e 26- 2
•Configuring QoS, p age 26-9
•Displaying QoS Information, page 26-28
•QoS Configurat ion Exa mple s, page 26-2 9

Understandin g QoS

This se ction de scri bes how QoS is im pl eme nted on the swit ch. If y ou have the SI insta ll ed on yo ur
switch, some concepts and features in this section might not apply. For a list of available features, see
Table 26-1 on pag e 26 -1.
Typ ically, netw orks operat e on a best-ef fort deli v ery basis, wh ich means th at all t raff ic h as equal priority
and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an
equal chance of being dr opped.
When you configure the QoS feature, you can select specific network traffic, prioritize it according to
its rel ative impor tance , and u s e c ongest ion- ma nage ment an d c on ges tio n-avoidance t ech nique s to
provide pr efe rent ial trea tme nt. Im ple menti ng QoS i n yo ur ne twork makes net work p er forma nc e mo re
predic tabl e and ban dwidt h utiliz ation more effect ive.
The Qo S i mple me nt at io n is bas ed o n th e D iffServ ar ch ite ct u re, a n em ergin g s tan da rd fr om the Inte rn et
Enginee ring Task Fo rce ( IETF). T his ar chitec ture spe cif ies that each pack et i s classi f ied u pon entr y into
the network. Th e classi fication is car ried in the IP pa cket header, using 6 bits from the de precate d IP
type-of-service (ToS) field to carry the classification (class) information.
Classification can also be carried in the Layer 2 frame. These special bits in the Layer 2 frame or
a Layer 3 packet ar e describe d here and shown in Figure 26-1:
•Prioritization values in Layer 2 frames
Layer 2 802. 1Q fr am e h eade rs h ave a 2-byte Ta g Co ntrol I nforma ti on field that car rie s the c lass o f
service (CoS) value in the three most-significant bits, which are called the User Priority bits. On
interfac es c onfigured a s L aye r 2 802. 1Q tru nks, al l tra ffic is in 80 2. 1Q f ra mes excep t fo r t raffic in
the native VLAN.
Other frame types cannot carry Layer 2 CoS values.
Layer 2 CoS values ra nge from 0 for low priority to 7 for high prior ity.
•Prioritization bits in Layer 3 packets
Layer 3 IP pac kets can carr y a Differentiat ed Servi ces Code Poin t (DSCP) value. The suppo rted
DSCP values a r e 0, 8, 1 0, 16, 1 8, 24 , 26, 3 2, 34, 40 , 46 , 48, and 56 .