Cisco Catalyst 3750 Software Configuration Manual page 505

Chapter 26 Configuring QoS
You configure the CIR and PIR rates in bps (or as a percentage of the bandwidth available on an ES port),
and these rates control how fast the bucket fills (is updated) with tokens. The conform burst size (bc) and
the peak burst size (be) represent the depth of the CIR and PIR buckets in bytes. This depth limits the
number of tokens that the bucket can accumulate. If the bucket fills to capacity, newly arriving tokens
are discarded.
Each token is permission for the source to send a certain number of bits into the network. To send a
packet, the number of tokens equal to the packet size must be drained from the bucket. If there are
enough tokens in the bucket, the packet conforms and can pass to the next stage. Otherwise, the exceed
action associated with the bucket is applied to the packet. The packet might be dropped, or its priority
value might be marked down.
In this token-bucket example, if the CIR rate is 2 kbps, 2000 tokens are added to the bucket every second
(for this example, consider each token to represent a single bit of information). If a 1500-byte packet
arrives, 12000 tokens (1500 bytes x 8 bits per byte) must be in the bucket for the packet to pass to the
next state without triggering the exceed action. If enough tokens are in the bucket, they are drained, and
the packet conforms and passes to the next stage. If there are less than 12000 tokens in the bucket, the
exceed action is applied to the packet. The deeper the bucket, the more data can burst through at a rate
greater than the rate at which the bucket is filling. For example, if the CIR bucket holds 6000 tokens, 750
bytes of traffic can instantaneously burst without draining the bucket (and without triggering an exceed
action), even though the instantaneous burst is at a greater rate than the CIR rate of 2000 bps.
If the burst sizes approach the system maximum transmission unit (MTU), the policer strictly enforces the
CIR and PIR. Normal traffic jitter can cause some percentage of inbound traffic to be flagged as
nonconforming even if the average inbound rate appears to conform. If the burst size is very large, on the other
hand, large traffic bursts at nonconforming data rates can be passed through the policer and flagged as
conforming. Setting the burst sizes too low can result in less traffic than expected, and setting them too
high can result in more traffic than expected.
For packet marking actions, if the CIR is 100 kbps, the PIR is 200 kbps, and a data stream with a rate of
250 kbps arrives at the two-rate policer:
• 100 kbps is marked as conforming to the rate.
100 kbps is marked as exceeding the rate.
•
50 kbps is marked as violating the rate.
•
If you set the CIR equal to the PIR, a traffic rate that is less than the CIR or that meets the CIR is in the
conform range. Traffic that exceeds the CIR rate is in the violate range.
If you set the PIR greater than the CIR, a traffic rate less than the CIR is in the conform range. A traffic
rate that exceeds the CIR but is less than or equal to the PIR is in the exceed range. A traffic rate that
exceeds the PIR is in the violate range.
After you configure the policy map and policing actions, attach the egress policy to an ES port by using
the service-policy output interface configuration command. For configuration information, see the
"Configuring Hierarchical QoS" section on page
78-15870-01
26-76.
Catalyst 3750 Metro Switch Software Configuration Guide
Understanding Hierarchical QoS
26-25