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.