Hi,
To understand the concept of burst, you have to understand how the router work work out whether traffic is within CIR or not.
Consider an example: CIR is 128000 bps and the conform-burst is 32000 bits. So what does that mean ? In order to ensure that your CIR does not exceed 128000 bps, the 32000 bits should be received in a time interval that is no less than 32000/128000 = 0.25seconds = 250 milliseconds. As long as the router receives less than 32000 bits in a 250 millisecond interval, the traffic will be conforming. So the burst parameter determines the granularity of policing. If the conform-burst was set to 128000 bits, the router would consider traffic to be conforming to the CIR if it received less than or equal to 128000 bits in a 1-second interval. Now let's say that the access (physical) rate of this interface is 512000 bps. Therefore the maximum rate at which a device can send traffic into this interface is limited to 512000 bps. If the burst is set to 32000 bits, this device can send traffic at 512000 bps for a period of 32000/512000 = 0.0625 seconds or 62.5 milliseconds. Since after 62.5 milliseconds have elapsed, the burst will exceed 32000, this device will now need to stop sending traffic until the next 250 millisecond interval starts (as worked out above). With a burst of 128000 bits, the device can send traffic at 512000 bps for a period of 128000/512000 = 0.25 seconds or 250 milliseconds. Since after250 milliseconds have elapsed, the burst will exceed 128000, this device will now need to stop sending traffic until the next 1000 millisecond interval starts (as worked out above).
So you see, the size of the burst determines how long a device can exceed the CIR before it needs to stop and take a break. IF the conform-burst is large, the device can exceed the CIR for a longer burst period, but it then has to take a longer break before it can burst again. If the conform-burst is smaller, the device can exceed the CIR for a lesser burst period, but it then has to take a shorter break before it can burst again.
Hope that helps - pls rate the post if it does.
Paresh
