Cisco’s recommendation is to leave the shaping values (Bc – committed burst and Tc – committed rate interval time) alone if you’re not servicing delay sensitive application, like voice. By default Class Based Shaping use 8000 bits for Bc and 125 ms for Tc if it is below 320 Kbps shaping rate. For 320 Kbps and above, the Tc changes to 25 ms and the Bc will be calculated based on the formula below. For example, the shaping rate is 640 Kbps, if you look at the formula, Bc = Tc * CIR. The CIR (Committed Information Rate) can be replaced by the shaping rate value. In this scenario, the shaping rate is 640,000 bits/sec. Going back to the example, since the shaping rate is over 320Kbps, we know that Tc is 25ms (0.025 secs) and if we multiply it to the shaping rate of 640 Kbps then we get 16 Kb for the Bc.
Let’s say for example that your shaping rate is 128 Kbps and you need to traverse a single G.711 phone call. As mentioned above, the Tc is 125 ms for shaping rate lower than 320 Kbps, this value is not an acceptable value for VoIP calls. The recommended one way delay is 150 ms. Allowing the default values of the shaping rate will make all voice calls unacceptable and you’ll start hearing complaints from users. Since we have the formula, we can manipulate the value to send packets every 10 ms to lessen the one way delay. If you look at the command below, you will see that Tc is not configurable. IOS calculates the Tc from the values of Bc and CIR/Shaping rate. Going back to the formula above, we know that we want to send packets every 10 ms and we know our CIR/shaping rate is 64 Kbps, let’s plug in the values to the formula to find out the Bc, which is needed to complete our command. Bc = 0.010 (Tc) * (CIR/Shaping rate) 128000 = 1280 bits. To check our math, please check the show policy-map output below.
R1(config-pmap-c)#shape average ?
Target Bit Rate (bits per second), the value needs
to be multiple of 8000
percent % of interface bandwidth for Committed information
rate
R1(config-pmap-c)#shape average 128000 ?
bits per interval, sustained. Needs to be multiple of
128. Recommend not to configure it, the algorithm
will find out the best value
For an overview of traffic shaping, please visit this site.
References
Cisco QoS Exam Certification Guide (IP Telephony Self-Study) (2nd Edition)
End-to-End QoS Network Design: Quality of Service in LANs, WANs, and VPNs
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