RE: Shaping and calculating Be value for a 30 second burst

Kambiz,

I totally agree with you. However, I must say that Cisco itself is the main
source of the confusion. As an example just look in the Cisco's
certification guide v4, page 597. In the key topics they say "Bc=CIR/32".
This is totally insane. At the top of the same page they even manage to
provide a "proof"!!! The "formula" Bc=CIR/32 clearly implies that units are
the same at each side of the equation, which is not the case! I learned in
the elementary school not to compare apples and oranges but this is exactly
what Cisco does here.

For me personally, there is only one formula "Bc=CIR*Tc". It is as obvious
as distance=speed*time. In the example above all you need to remember is
that Tc=0.25 second. With *understanding* of the principle anyone should be
able to figure out the rest. No need for confusing Cisco explanations.

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
Kambiz Agahian
Sent: 25-Apr-10 15:44
To: Ladee Geek; CCIE Study Group
Subject: RE: Shaping and calculating Be value for a 30 second burst

Hi Ladee,

I guess you need to revise some parts of your understanding and take a fresh
look at the example - I never give a YES/NO answer but I want you to
*understand* it:

Fact: Be is not AIR-CIR. Why? Because Be basically represents a "quantity"
and
not a "rate". Both AIR and CIR are sort of rates (say bit/s) and the output
of
a subtraction operation won't change the "units".

If you ask legendary network engineers (60+ y.o!) they'd say "Bc is CIR per
TC
and Be is AIR-CIR per Tc". Actually their training was simple and
effective...hence again you can say both Bc and Be are referring to the
number
of bits and not rates. Technically it's true to say my Bc or my Be is 32000
bits but it's not true when people say hey the Bc is now 32Kbps.

Many of my students just try to memorize things like Bc=CIR*Tc etc. but when
you ask them "WHY?" they're always like "ummm hang on...it was..that was..."
Why? because no one had enough time to draw a simple primary school-like
diagram to prove it. Would you like to be different? spend a couple of hours
and take a "math look" at the QoS formulas.

Problem?
That bloody bucket!!! whenever we start from "the bucket" after 2-3 mins
students stop learning! but if you think about the math behind the scene,
it's
just a piece of cake.

You might wish to see these:
http://www.cisco.com/en/US/tech/tk543/tk545/technologies_q_and_a_item09186a0
0
800cdfab.shtml
http://www.cisco.com/en/US/tech/tk543/tk545/technologies_tech_note09186a0080
0
a3a25.shtml

--------------------------
Kambiz Agahian
CCIE (R&S)
CCSI, WAASSE, RSSSE
Technical Instructor
CCBOOTCAMP - Cisco Learning Solutions Partner (CLSP)
Email: kagahian_at_ccbootcamp.com
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-----Original Message-----
From: nobody_at_groupstudy.com on behalf of Ladee Geek
Sent: Sun 4/25/2010 12:10 PM
To: CCIE Study Group
Subject: Shaping and calculating Be value for a 30 second burst

I have a question about determining the Be for generic traffic shaping.
I've looked through the QoS exam guide, QoS configuration guide, the doc cd
and haven't seen enough to be able to figure this one out.

AR - 1.544 mb
Cir - 768k
Tc - 125 ms
burst for 30 sec up to line rate

I understood Be to be AR - Cir.

The answer key for this task states that the Be is calculated by taking
the line rate and multiplying it by 30 sec?

1544000bps*30s = 46320000bits

There's just no way to put that many bits on the wire in 1 sec. Or is the
Be the number of tokens to be replenished in 1 sec ( same as 5790000
bits/Tc)

And while I am at it, if you've found a good reference please pass it on.

--
r/
LG
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