# QOS average shaping BC AND BE

Oct 30th, 2009

I'm trying to understand how can the average shaping have a BE parameter? I thought average shaping does no allow for bursting.

shape {average | peak} cir [bc] [be]

Any help would be greatly appreciated.

Thank you,

TK

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## Replies

thotsaphon Fri, 10/30/2009 - 13:04

Tomasz,

First off, you must know what the paramters are.

- Line Rate/Access Rate. Let's say AR is 512 Kbps.

- CIR. Let's say CIR is 256 Kbps. This is what you want to shape.

- TC. Interval Time that you can send the packets. It's 1/8 second if BW of the link is under 768. If I recall.

- BC = Cir x TC

- BE = (AR - CIR) x TC. Yes. You can burst it if you didn't send packets in some time interval .

HTH,

Toshi

tievens Wed, 12/02/2009 - 12:34

Average only includes Bc while peak includes Bc+Be.  In other words, if you use average then only CIR and Bc are used.

If Bc is in bits, use the following to calculate the Bc value.

Bc=CIR * (ms/1000)

Example:

4ms is the minimum Tc value in IOS, which means CIR * .004 is the lowest value you can use for Bc.

10Mbps would be the following using a 4ms Tc

10000000 * .004 = 40000

shape average 10000000 40000

The lower the Tc the more smooth the traffic will be (shaped).  This can also drive the CPU up a bit higher, so you should use caution if your CPU is already running high.

HTH

--Tim