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

Port Priority

Frames received from users in the administratively-defined VLANs are classified or tagged for
transmission to other devices. Based on rules that you define, a unique identifier (the tag) is inserted in
each fr ame head er before it is forw arded. The tag is e xamined and under stood b y each de vic e befor e any
broadca sts or tra nsm issions t o ot her sw itch es , rout ers, or en d st atio ns. When th e f rame rea ches th e la st
switch or router, the tag is removed before the frame is sent to the target end station. VLANs that are
assign ed o n trunk o r a cce ss ports w ith out iden t ification or a ta g ar e ca lled native or untagged frames.
For IEEE 802.1Q frames with tag information, the priority value from the header frame is used. For native
frames, the default priority of th e inp ut port is used.

Port Scheduling

Each p ort on the swit ch ha s a si ngle rec eive queue buffer (the ing ress port ) for incom ing traffic. When
an untagged frame arrives, it is assigned the value of the port as its port default priority. You assign this
value by using the CLI or CMS. A tagged frame continues to use its assigne d CoS value when it passe s
through the ingress port.
CoS co nfigures eac h tran smi t port (t he egress port) with a normal-priority transmit queue and a
high-pr iority tra nsmit que ue, depe nding on the fra me tag or the port info rmation. Frames in the
normal-priority queue are forwarded only after frames in the high-priority queue are forwarded.
The switch (802.1P user priority) has four priority queues. The frames are forwarded to appropriate
queues based on the p rior ity-t o-que ue m ap ping tha t yo u d efined.

CoS and WRR

The swi tch suppo rts fou r CoS qu eue s fo r eac h egres s port. For each que ue, yo u ca n spec ify t hes e typ es
of sche duling :
•Strict priority scheduling
Strict priority scheduling is based on the priority of queues. Queues can hav e priorities from 0 to 7,
7 bein g th e high est. Packets in the h igh- prior ity q ueue a lways tra nsm it first, an d packet s in th e
low-priority que ue do not tran smit unt il all the high -priori ty queue s become empty.
•Weigh ted round- robin (WRR ) schedu ling
WRR scheduling requires you to specify a number that indicates the importance (weight) of the
queue rela tive to the othe r CoS queues. WRR scheduling prevents the low-p riority queues from
being compl et ely negl ect ed d urin g p er iods of high -pri ority t raffic. T he WR R sche dule r t ransm its
some pa ckets from e ach queue in tu rn. Th e numbe r of pa c kets it se nd s corr e spon ds to the r elat ive
import ance of the queue. For example, if one que ue has a weigh t of 3 and anot her has a weight of
4, three pa ckets are sent from the first queue for every four that are sent fro m the second queue. By
using this sch edul ing, low-prior ity queu es have the opp ortuni ty to send pa ckets even thou gh the
high-pr iority queue s are not em pty.