Thursday, 12 April 2012

QoS Shaping Calculations

Cisco IOS has some default for shaping and policing. These are defaults are based on the CIR.

Committed Information Rate (CIR) (the shaping rate)
Committed Rate Measurement Interval (Tc)
Committed Burst (Bc)
Excess Burst (Be) 

Generic Traffic Shaping (GTS) & CB shaping default var settings:

  • Bc: 8000 bits if rate <= 320kbps, Bc=shaping rate*Tc if > 320 kbps
  • Be: Be=Bc= 8000 bits if rate <= 320kbps, Be=Bc if > 320 kbps
  • Tc: Tc = Bc/shaping rate if rate <=320 kbps, Tc=25ms if > 320kbps

You can simplify this as follows

If Rate <=320kbs
  • Bc = 8000 bits
  • Be = 8000 bits
  • Tc = 8000/shaping rate

If Rate >320kbps
  • Bc = shaping rate * 0.025
  • Be = Bc
  • Tc = 0.025

When manually setting the Bc & Be parameters, a big deciding factor is the most used/important application. For data applications doing large file transfers, a larger Tc is generally recommended. For voice you want the smallest possible Tc, to avoid voice packets having to wait a large amount of milliseconds for the next interval before being sent.


A router is connected to an HDLC circuit via a T1 physical interface. The SLA for this link only allows for a sustained rate of 768 kb/s. Bursts are allowed for up to 30 seconds at up to line rate, with a window Tc of 125 ms.
What should the Be and Be setting be when using generic traffic shaping?

the sustained rate is the CIR = 768kb/s
From the formula Tc=Bc/CIR => Bc = Tc * CIR = 125ms * 768kb/s = 96000 bits
(In fact you should calculate with the default units, that is 0.125s * 768000b/s)

The T1 speed is 1.544 Mbps = 1544000bps. “Bursts are allowed for up to 30 seconds at up to line rate” ->Be = 1544000bps * 30 = 46320000 bits.


  • The term CIR refers to the traffic rate for a VC based on a business contract.
  • Tc is a static time interval, set by the shaper.
  • Committed burst (Bc) is the number of bits that can be sent in each Tc.
  • Be is the excess burst size, in bits. This is the number of bits beyond Bc that can be sent after a period of inactivity.

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