Shape peak vs Shape Average

The shape peak replenishes both Bc and Be tokens every Tc seconds, and allows for sending at higher sustained rate that depends both on the size of Bc and Be. If only the Bc tokens were replenished, the sustained rate would be

CIR = Bc/Tc

With both Bc and Be tokens being replenished in shape peak, the sustained rate is

CIR' = (Bc+Be)/Tc

Expressing Tc from the first equation yields Tc=Bc/CIR, and using it in the second equation yields:

CIR' = (Bc+Be) / (Bc / CIR)=(Bc+Be) * CIR / Bc = CIR * (1 + Be/Bc)

This is the speed as indicated in the Command Reference for the shape peak command:

http://www.cisco.com/en/US/docs/ios/qos/command/reference/qos_s1.html#wp1074988

The shape average replenishes only the Bc tokens every Tc seconds, and hence the sustained rate is simply CIR=Bc/Tc. After periods of longer inactivity, the burst may grow up to Bc+Be bytes but it is not sustainable.

To compare:

• The sustained rate in shape average is equal to CIR=Bc/Tc (the CIR is a part of the configuration command)
• The sustained rate in shape peak is equal to CIR'=CIR * (1 + Be/Bc) (the CIR is a part of the configuration command)
• Both models allow for bursting up to Bc+Be

In default configuration, Be=Bc, therefore with shape peak, the CIR'=CIR*(1+Bc/Bc)=CIR*(1+1)=2*CIR, i.e. theshape peak would result into shaping on a doubled sustained rate than the shape average command.