Shaping increments the token bucket at timed intervals using a bits per second (bps) value. A shaper uses the following formula:
In this equation, Bc represents the committed burst, and CIR stands for committed information rate. (See Configuring Frame Relay Traffic Shaping for more information.) The value of Tc defines the time interval during which you send the Bc bits in order to maintain the average rate of the CIR in seconds.Tc = Bc/CIR (in seconds)
When the excess burst (Be) is configured to a value different than 0, the shaper allows tokens to be stored in the bucket, up to Bc + Be. The largest value that the token bucket can ever reach is Bc + Be, and overflow tokens are dropped. The only way to have more than Bc tokens in the bucket is to not use all Bc tokens during one or more Tc. Since the token bucket is replenished every Tc with Bc tokens, you can accumulate unused tokens for later use up to Bc + Be.
In contrast, class-based policing and rate-limiting adds tokens continuously to the bucket. Specifically, the token arrival rate is calculated as follows:
In other words, if the previous arrival of the packet was at t1 and the current time is t, the bucket is updated with t-t1 worth of bytes based on the token arrival rate. Note that a traffic policer uses burst values specified in bytes, and the above formula converts from bits to bytes.(time between packets<which is equal to t-t1>* policer rate)/8 bits per byte
Shaping implicitly uses the averaging interval Tc=Bc/CIR to meter the average traffic rate. Larger Bc values allow for better accommodation of bursty traffic flows with oscillating traffic rates. Smaller Bc is good for stable flows, in situations where you don’t tolerate even a small violation of the instant traffic rate.
Ref:
The Meaning of Bc with Traffic Policing
Comparing Traffic Policing and Traffic Shaping for Bandwidth Limiting
Shaping and policing on Cisco
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