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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter 20 Configuring QoS
Understanding QoS
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 20-8
.
A policy map also has these characteristics:
•
A policy map can contain multiple class statements.
•
A separate policy-map class can exist for each type of traffic received through an interface.
•
The policy-map trust state and an interface trust state are mutually exclusive, and whichever is
configured last takes affect.
For configuration information, see the
“Configuring a QoS Policy” section on page 20-26
.
Policing and Marking
After a packet is classified and has an internal DSCP value assigned to it, the policing and marking
process can begin as shown in
Figure 20-4
.
Policing involves creating a policer that specifies the bandwidth limits for the traffic. Packets that 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 passing through the packet
without modification, dropping the packet, or marking down the packet with a new DSCP value that is
obtained from the configurable policed-DSCP map. For information on the policed-DSCP map, see the
“Mapping Tables” section on page 20-11
.
You can create these types of policers:
•
Individual
QoS applies the bandwidth limits specified in the policer separately to each matched traffic class.
You configure this type of policer within a policy map by using the police policy-map configuration
command.
•
Aggregate
QoS applies the bandwidth limits specified in an aggregate policer cumulatively to all matched
traffic flows. You configure this type of policer by specifying the aggregate policer name within a
policy map by using the police aggregate policy-map configuration command. You specify the
bandwidth limits of the policer by using the mls qos aggregate-policer global configuration
command. In this way, the aggregate policer is shared by multiple classes of traffic within a policy
map.
Policing uses a token bucket algorithm. As each frame is received by the switch, a token is added to the
bucket. The bucket has a hole in it and leaks at a rate that you specify as the average traffic rate in bits
per second. Each time a token is added to the bucket, the switch performs a check to determine if there
is enough room in the bucket. If there is not enough room, the packet is marked as nonconforming, and
the specified policer action is taken (dropped or marked down).
How quickly the bucket fills is a function of the bucket depth (burst-byte), the rate at which the tokens
are removed (rate-bps), and the duration of the burst above the average rate. The size of the bucket
imposes an upper limit on the burst length and determines the number of frames that can be transmitted
back-to-back. If the burst is short, the bucket does not overflow, and no action is taken against the traffic
flow. However, if a burst is long and at a higher rate, the bucket overflows and the policing actions are
taken against the frames in that burst.
