Specifications

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on the number of bits in the packet (i.e. 512 bytes x 8 bits = 4096). If only 4095 tokens are in the bucket then the

packet cannot be sent (it is dropped) and the tokens remain in the bucket. When a packet is sent, the tokens are

removed from the bucket.

The depth of the bucket is another important factor to consider. The Burst should be set so that it can hold the

largest sized packet in the data flow being policed. In IOS, the minimum defined burst size is 1000. The number

1000 equates to a burst size of 1 kilobyte or the buckets ability to hold 8000 tokens (i.e. 1 kilobyte = 8000 bits). If this

value were used, and the data flow consisted of maximum size Ethernet packets (1518 byte), then no packet could

ever be sent as there would never be enough tokens in the bucket to forward a packet in each interval. The Burst

should be set to a value that ensures the switch can at least sustain the configured Rate. If a burst value is defined

in the policer, the minimum set will be equal to the rate in a 4-millisecond interval. For example, if a 10Mbps rate

were specified, then the minimum burst would be 5000 which is calculated as follows:

10,000,000 bits x 0.004 seconds / 8 bits = 5000

This is confirmed showing the following CLI where a lower burst of 4000 is configured:

c6500(config-pmap-c)# police 10000000 4000 conform transmit exceed drop

Info: Illegal normal burst size, increased to 5000

If no burst is specified, then the default burst will be calculated at 250ms. In the

above example, this would set the burst to 312500 which is calculated as follows:

10,000,000 x 0.25 seconds / 8 bits = 312500

This can be confirmed as shown by the following CLI

c6500(config-pmap-c)# police 10000000 conform transmit exceed drop

c6500(config-pmap-c)#^Z

c6500# show policy

Policy Map limit_to_10Mb

Class marketing

police 10000000 312500 312500 conform-action transmit exceed-action drop

Note the resulting burst value in the show output. As a fallback, always remember that the CLI will reset your

configured Burst size if it is too small for the configured Rate.

8.8 A Walk through how the Token Bucket is used

To better explain how the token bucket works, a simple example will be explored below.

Step 1: At the beginning of the time interval (call it T0), the token bucket is primed with a full complement of tokens.

This is always the case whenever a policer is activated.

Step 2: Data will start to arrive at the switch. The policer will only process data that fits within the classification

criteria and arrives in the given 0.25ms interval. Any data that has not fully clocked in within that interval will be

processed in the next interval.