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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

Classification Based on Class Maps and Policy Maps

A class map is a mechanism that you use to isolate and name a specific traffic flow (or class) from all
other traffic. The class map defines the criteria used to match against a specific traffic flow to further
class ify it; the crite ria can inc lude matc hing th e access gr oup def ined b y the A C L. If you ha v e more tha n
one type of traffic that you want to cla ssify, you can crea te an ot her cl ass ma p and use a differen t name .
After a packet is matched against the class-map criteria, you further classify it through the use of a policy
map.
A policy map specifies which traffic class to act on. Actions can include setting a specific DSCP value
in the traffic class or specifying the traffic bandwidth limitations and the action to take when the traffic
is out of profile. Before a policy map can be effective, you must attach it to an interface.
You crea te a class ma p by using the class-map globa l c onfigurati on c om mand or the class policy-map
configuration command. You should use the class-map global configuration command when the map is
shared am ong m any po rts. Wh en yo u ent er t he class-map global configurat ion c omma nd, t he swit ch
enter s th e cl ass-ma p c onfigura tio n mod e. In this m ode , y ou define the m atc h cri ter ion for t he traffic by
using the match class-map co nfigurat ion co mma nd.
You create and name a policy map by using the policy-map global c onfigura tio n com ma nd. Wh en you
enter this c ommand, the switc h enters th e polic y-map confi guratio n mode. I n this mode, y ou specify the
actions to take on a specific traffic class by using the class policy-m ap co nfigur atio n or set p o licy-ma p
class configuration command. To make the policy map effective, you attach it to an interface by using
the service-policy interface configuration command.
The policy map can also contain commands that define the policer, the bandwidth limitations of the
traffic, and the action to take if the limits are exceeded. For more information, see the “Policing and
Marking” section on page 26-6 .
A policy map also has these characteristics:
•A pol icy ma p c an co nt ain mu l tiple c la ss sta t emen ts .
•A separate policy-map class can exist for each type of traffic received through an interface.
•A policy-map configuration state supersedes any actions due to an interface trust state.
For configuration information, see the “Configuring a Q oS Policy” section on page 26-16.

Policing and Marking

Note Th is fea ture is available o nly i f your sw itch i s r unn ing the EI .
Policing i nv olves c reating a policer that specifie s the bandwidth lim its for the traf fic. Packets th at exceed
the limits are out of profile or nonconforming. Each policer specifies the action to take for packets that
are in or out of profile. These actions, carried out by the marker, include dropping the packet or marking
down the packet with a new user-defined value.
You can creat e an indiv idual policer . Qo S applies the bandwidth limits specifie d in the policer separatel y
to each matched traffic class. You configure this type of policer within a policy map by using the
policy-map configuration command.
When configuri ng policin g and police rs, keep these i tems in mind :
•By default, no policers ar e configured .
•Policers can only be configured on a physical port. There is no support for policing at a VLAN level.