26-4
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapt er 26 Conf i gurin g QoS
Unders tanding QoS
Marking evaluates the poli cer and configuration informati on for the action to be taken when a packet
is out of profile and decides what to do with the packet (pass through a packet without modification,
mark down the DSCP value in the packet, or drop the packet). For more information, see the
Policing a nd Mark ing sect ion o n page 26-6.
Action s a t the egre ss in ter face incl ude qu eu eing a nd sc he duling:
Queuei ng e v aluates the CoS v alue an d determi nes which of the four egr ess queues in wh ich to place
the packet.
Scheduling services the four egress queues based on their configured weighted round robin (WRR)
weights.
Figure 26-2 Basic QoS Model

Classification

Note Th is fea ture is available o nly i f your sw itch i s r unn ing the EI .
Classification is the proce ss of distingu ishing one kind of traffic from another by examinin g the fields
in th e pa cket.
Classification occurs only on a physical interface basis. No support exists for classif ying packets at the
VLAN level.
You spe cify w hic h fields in the fra me or pa cket that you want t o use to c lassi f y in co ming tra ffic.
For non-IP traffic, you have these classification opt ions :
Use the port default. If the frame does not contain a CoS value, the switch assigns the default port
CoS value to the incoming frame.
Trust the CoS value in the incoming frame (configure the port to trust CoS). Layer 2 802.1Q frame
headers carry the CoS value in the three most-significant bits of the Tag Control Information field.
CoS values range fr om 0 for low priority to 7 for high prior ity.
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Classification Policing
Actions at ingress Actions at egress
Mark
In profile or
out of profile
Classify the packet
based on the ACL. Determine if the
packet is in profile or
out of profile based
on the policer
associated with the
filter.
Based on whether
the packet is in or
out of profile and the
configured
parameters,
determine whether
to pass through,
mark down, or drop
the packet. The
DSCP and CoS are
marked or changed
accordingly.
Queueing and
scheduling
Based on the CoS,
determine into which
of the egress
queues to place the
pack et. Then service
the queues
according to the
configured weights.