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Catalyst 3750 Metro Switch Software Configuration Guide
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Chapter 26 Configuring QoS
Understanding QoS
A single-rate traffic policer decides on a packet-by-packet basis whether the packet is in or out of profile
and specifies the actions on the packet. These actions, carried out by the marker, include passing through
the packet without modification, dropping the packet, or modifying (marking down) the assigned DSCP
of the packet and allowing the packet to pass through. The configurable policed-DSCP map provides the
packet with a new DSCP-based QoS label. For information on the policed-DSCP map, see the “Mapping
Tables” section on page 26-11. Marked-down packets use the same queues as the original QoS label to
prevent packets in a flow from getting out of order.
Single-rate 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 bps. 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 sent 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 ingress policy to a port by using the
service-policy input interface configuration command. For configuration information, see the
“Classifying, Policing, and Marking Ingress Traffic by Using Policy Maps” section on page 26-54 and
the “Classifying, Policing, and Marking Ingress Traffic by Using Aggregate Policers” section on
page 26-57.
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