BC and BE

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Nov 16th, 2008
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Can someone explain me in detail about bc & be? How does both of them differ in practical scenario?

Whats the difference b/w exceed action and violate action?

police cir 512000 bc 50000 be 80000 conform-action transmit exceed-action drop violate-action drop

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Overall Rating: 4.5 (2 ratings)
Mohamed Sobair Sun, 11/16/2008 - 23:19
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The (Bc) means commited burst which = CIR + Bc

The (Be) means Excess Burst which = CIR + Be

the (Exceed Action) is conducted when going above the Bc.

The (Violate action) is conducted when exceeding the Be.

The Conform Action is conducted when commited to the CIR or between 0 --to-- CIR value.



aneesh.ts Sun, 11/16/2008 - 23:23
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Hi Mohammad,

Thank you very much for the reply. But my confusion is regarding Excess Burst and Committed burst. Is there any formulae to calculate BC and BE from the CIR. Also practically why there are two kinds of bursts. Whats the difference between both of them traffic-wise?

aneesh.ts Mon, 11/17/2008 - 00:19
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So is it like what I have put down below. If wrong please correct.

0 - CIR+BC ----> Confirm Action

> CIR+BC - CIR+BE ------> Exceed Action

>CIR+BE -----> Violate Action

Mohamed Sobair Sun, 11/16/2008 - 23:32
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As the Name implies:

(Bc) or (Commited Burst) is the commited allowed burst value by the Service provider.

(Be) or (Excess Burst) is the Excess burst size which is not allowed by the Service Provider.

Practically, Setting the Be could be of risk because traffic will be dropped when congestion occurs.



aneesh.ts Mon, 11/17/2008 - 00:57
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When we add PIR to the previous configuration which is mentioned below, what difference does it make.

police cir 128000 bc 50000 pir 512000 be 80000 conform-action set-mpls-exp-transmit 1 exceed-action transmit violate-action drop

What comes under conform-action, exceed action and violate-action?

Mohamed Sobair Mon, 11/17/2008 - 01:53
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Please have a look at each paragraph:

Committed Information Rate (CIR)

This is the rate (expressed in kbits/s) which is guaranteed that will always be available to the respective traffic class. Apparently, the CIF dedicated for a specific class, can not exceed the network bandwidth available. When multiple competing classes exist for the same interface and for the same direction (output/input), the sum of all of them should also not overrun the available bandwidth.

Note that, regardless of the CIR the traffic is always transmitted at the maximum speed supported by the physical interface. Literally, the CIR expresses the average rate in which the traffic is sent, in due time.

Peak Information Rate (PIR)

This is the maximum rate (in kbits/s) in which, the traffic of a class, can be sent or received (in average). Even if no other traffic competes for the bandwidth, this barrier can not be exceeded. This value can be as large as the capacity of the link and as small as the CIR.

The bandwidth between CIR and PIR is not guaranteed for a class. The possibility for a class to exploit this range, depends on its priority as we will see later.

Excess Burst Size (EBS)

Some applications are characterized by short periods of intensive network usage and long periods with no network usage at all. For instance, when we browse the Internet, our web browser requests a web page and then remains idle for a long period of time, until another page is requested. Such applications are not served well by the CIR/PIR mechanism alone. The EBS mechanism remedies this problem by allowing an application to send a number of bytes continuously, for some time, without being interrupted. As soon as EBC bytes have been sent, the application is forced back to normal behavior (average rate ranging between CIR and PIR).

Committed Burst Size (CBS)

The CBS corresponds to the minimum number of bytes that have to be available in order for a transmission to start. By the time that the transmission starts, it is not possible to be interrupted, until there are no other data to send. By default this value is the smallest possible (a single packet size ideally) and scarcely will you have to set a different value.

In order to better understand the concept of rate and burst, consider the analogy: Each class (or subclass as we will see later) is like a bucket with size EBS. The bucket is filled up at a rate which ranges between CIR and PIR. In accordance with this analogy, transmission starts when we throw water out of the bucket. The minimum quantity of water (traffic) that we can be thrown out is CBS. Therefore, when a class is idle for a while, it's possible for an application later on, to send a large burst of data, until the “bucket” is empty. Similarly, for a class that sends traffic at a steady rate, lower than CIR, its “bucket” will always be filled up




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