Cisco WS-C3560-48PS-S Software Configuration Manual page 531

Chapter 28 Configuring QoS
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.
After you configure the policy map and policing actions, attach the policy to an ingress port by using the
service-policy interface configuration command. For configuration information, see the
Policing, and Marking Traffic by Using Policy Maps" section on page 28-42
Policing, and Marking Traffic by Using Aggregate Policers" section on page
Figure 28-4 Policing and Marking Flowchart
Get the clasification
result for the packet.
Is a policer configured
Check if the packet is in
profile by querying the policer.
78-16156-01
Start
No
for this packet?
Yes
No
Yes
Pass
through
Check out-of-profile action
configured for this policer.
Modify DSCP according to the
policed-DSCP map. Generate
a new QoS label.
Done
Drop
Drop packet.
Mark
Catalyst 3560 Switch Software Configuration Guide
Understanding QoS
"Classifying,
and the "Classifying,
28-45.
28-9