Cisco QoS Exam Certification Guide by Wendell Odom

Author:Wendell Odom
Language: eng
Format: epub

QoS ECG.book Page 347 Tuesday, October 19, 2004 1:40 PM

Traffic Policing and Traffic Shaping Concepts 347

Figure 6-5

Mechanics of Traffic Shaping—128-kbps AR, 64-kbps Shaped Rate

Send packets for 62.5 ms per Interval (1/2 the time) in Order to Average 64 kbps

So Bc = 8000 bits (1000 bytes)

Sending Rate

128 kbps

0 kbps

0

125

250

375

500

625

750

875

1000

Time

(Ms)

The router should send literally half of the time to average sending 64 kbps on a 128-kbps link.

Traffic shaping accomplishes this by sending up to half of the time in each Tc.

As shown in the figure, R1 sends at line rate for 62.5 ms, and then is silent for 62.5 ms, completing the first interval. (The Tc defaults to 125 ms for many shaping tools; CB Shaping happens to default to another Tc in this case, but the concept is still valid.) As long as packets are queued and available, R1 repeats the process during each interval. At the end of 1 second, for instance, R1 would have been sending for 62.5 ms in 8 intervals, or 500 ms—which is .5 seconds. By sending for half of the second at 128 kbps, R1 will have sent traffic at an average rate of 64 kbps.

IOS traffic shaping does not actually start a timer for 62.5 ms, and then stop sending when the timer stops. IOS actually calculates, based on the configuration, how many bits could be sent in each interval so that the shaped rate would be met. This value is called the committed burst (Bc) for each interval. It is considered a burst, because the bits actually flow at the physical line rate. The burst is committed, because if you send this much every interval, you are still conforming to the traffic contract. In this example, the Bc value is set to 8000 bits, and the actual process allows the shaper to send packets in each interval until 8000 bits have been sent. At that point, the shaper waits until the Tc has ended, and another interval starts, with another Bc worth of bits sent in the next interval.

With an interval of 125 ms, and 8000 bits per interval, a 64-kbps shaped rate is achieved.

The Bc value is calculated using the following formulas:

Bc = Tc * CIR

or

Bc = Tc * Shaped rate

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