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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 39 Configuring QoS
Understanding QoS
Policing on Physical Ports
In policy maps on physical ports, 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
class 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 class 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.
Note You can only configure individual policers on an SVI.
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 verifies that 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 limits 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.
You configure the bucket depth (the maximum burst that is tolerated before the bucket overflows) by
using the burst-byte option of the police policy-map class configuration command or the mls qos
aggregate-policer global configuration command. You configure how fast (the average rate) that the
tokens are removed from the bucket by using the rate-bps option of the police policy-map class
configuration command or the mls qos aggregate-policer global configuration command.
Figure 39-4 shows the policing and marking process when these types of policy maps are configured:
• A nonhierarchical policy map on a physical port.
• The interface level of a hierarchical policy map attached to an SVI. The physical ports are specified
in this secondary policy map.