From: Mark Olson (molson@xxxxxxxxxx)
Date: Wed Oct 20 1999 - 12:05:52 GMT-3
Mike Wang wrote:
> Hi,
>
> I am sure you guys read the frame relay traffic shaping samples on CCO. I
> found something need to discuss with you regarding be calculation.
>
> The first example is cir 64000, bc 8000, mincir 16000, be 0. I understand
> be=port speed - cir =0
Well, you can't really come up with a formula for Be without making assumptions
about some of the other parameters. Cisco also makes it a little confusing wit
h
the parameter names they use. According to Cisco's terminology,
CIR = Average rate (better to say max rate) that you will transmit. You will
typically
set this to port speed.
MINCIR = This is what everyone else calls CIR. It's the amount of data that
will
be sent during congestions (ie, router receives BECNs).
Tc = time interval = Bc/CIR. In Cisco's world, this will not exceed 125 ms.
Bc = the amount of sent per Tc. Typical rule of thumb is Bc = 1/8 of CIR
(Cisco's CIR
that is!).
Be = amount of excess data sent during the FIRST time period.
As near as I can tell by messing around in the lab, the CIR parameter is really
only used to calculate Tc. By messing around with Bc and Be, you can actually
send more data than the CIR value.
So, taking the above example, Tc = 8/64 = 1/8 = 125 ms. So, here's what you'll
be
sending every 125 ms:
Time Period 1: 8 K (Bc + Be. Cisco will send the excess data in the first Tc)
.
Time Period 2: 8 K (Bc)
Time Period 3: 8K (Bc)
Time Period 4: 8K (Bc)
Time Period 5: 8K (Bc)
Time Period 6: 8K (Bc)
Time Period 7: 8K (Bc)
Time Period 8: 8K (Bc)
So, every second, you'll be sending 64 K worth of data. The 'sh traffic-shape'
command is your friend.
The value of all of these parameters are spelled out in the output of that
command.
>
>
> However, the second example is cir 64000, mincir 16000, bc 8000, be 72000. I
> do not understand this one: be should = port speed (128000)-cir (64000)
> =64000. The answer 72000 is obviously added bc. So my question is: should
> be include bc? My answer is be should be 64000.
>
Well, this is a totally bogus example. If you config your router for this and
do
a 'sh traffic-shape' command, You'll see that you would actually be sending 136
Kbps
worth of data. The mistake is with the Be value which should be 64 K instead o
f
72 K. Then you'd be sending 128 K worth of data.
Good Luck,
Mark
>
> Thanks,
> Mike
>
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