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.

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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.

Terminologies:

- 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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