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8.3 Rate and Burst
Two key parameters used implicitly in the configuration of policing are the Rate and Burst. The rate (also referred to
as the Committed Information Rate—or CIR) is defined as the maximum amount of data that can be forwarded in a
given interval (normally referred to in Kbps or Mbps). The burst can be thought of as the total amount of data that
can be received in a given interval. A simple example of how these parameters interact could be a policy that uses a
Rate of 10Mbps and a Burst of 15Mbps. This policy defines that a maximum of 15Mbps can be received in a given
interval from which 10Mbps (the Rate) of data can be sent. Relating this back to the bucket concept, the Burst can
be thought of as the depth of the bucket (how much data can be received), while the Rate can be thought of as a
hole in the bucket defining how much data can be leaked out of the bucket (or forwarded).
An important point to stress is that the Burst should “NEVER” be less than the stated Rate. If, for argument sake, the
Burst were set at 8Mbps, then a Rate of 10Mbps would be impossible to achieve. If the bucket can only ever hold
8Mb, the maximum Rate could only ever be 8Mbps as well.
8.4 PIR and Max Burst
PIR (Peak Information Rate) and the Max Burst are the next set of parameters that must be understood. While the
Rate and Burst are associated with the first bucket, the PIR and Max Burst are associated with the second bucket.
The Max Burst defines the depth of the second bucket, while the PIR is the amount of data that can be forwarded
from second bucket
8.5 Hardware Interval
The use of the term “given interval” above relates to a specific hardware defined interval built into the Catalyst 6500
Policy Feature Card hardware. A fixed interval of 1/4000th of a second, or 0.25 milliseconds is used in the policing
calculation. The hardware interval bears relevance to the replenishment rate of tokens into the token bucket. This is
discussed later in this paper. Another key difference is that while Rate is specified in bits, the burst is specified in
bytes (NOT BITS).
Figure 32. Policing Per Interval
As can be seen in the above diagram, data begins to arrive at the switch in a given interval and the data will be
forwarded as long as the total count of data is within the defined Rate (for that interval). Once the data count for a
given interval exceeds the Rate limit, the data will not be forwarded until the next hardware interval starts.