RE: Shaping and calculating Be value for a 30 second burst from Kambiz Agahian on 2010-04-30 (Ccielab archives 04/2010)

From: Kambiz Agahian <kagahian_at_ccbootcamp.com>
Date: Thu, 29 Apr 2010 23:51:05 -0700

Jack,

Math, bloody math and aah you goddamn math! Again we can blame this
(so-called) science...I loved it when I was a student though.

First off, thanks for chasing this up, I hate yes/no candidates and I'm
so glad to see someone like you in here who's interested in "WHY's" more
than just "HOW's".

Actually, Odom is one of the greatest Cisco press authors and accidently
he's a QoS specialist more than a routing/switching guy. He's also well
known for his nice examples in his classes...

Anyway, let's go back to the Cisco's certification guide v4, page 597
and see what's going on. You're absolutely right it's not probably the
best way to represent a numeric value but let's take a closer look; that
equation includes 3 parts; the Bc itself which is what we're after, then
a bit of stuff in the middle and that odd thing at the end.

You see the second part? { CIR*0.25 second } ; what is the "unit" of the
output of this statement? { Bit/sec * sec } ; so it yields "bits" hence
you can perfectly and safely divide it by 32 and say hey it's my Bc.

Odom however has done a better job in his QoS book and I strongly
recommend that you spend some time on that precious book well before
your lab date.

If you still need more clarification on how to "prove" the nature of Bc
(from a math's point of view) I have enough time to draw something and
get it to you...

Please let me know if you have any other issues with this.

--------------------------
Kambiz Agahian
CCIE (R&S)
CCSI, WAASSE, RSSSE
Technical Instructor
CCBOOTCAMP - Cisco Learning Solutions Partner (CLSP)
Email: kagahian_at_ccbootcamp.com
Toll Free: 877-654-2243
International: +1-702-968-5100
Skype: skype:ccbootcamp?call
FAX: +1-702-446-8012
YES! We take Cisco Learning Credits!
Training And Remote Racks: http://www.ccbootcamp.com
OEQ Voice Waiver: http://www.ccbootcamp.com/noeqvoice.html
OEQ R&S Waiver: http://www.ccbootcamp.com/noeqrs.html
OEQ Commercial: http://www.ccbootcamp.com/noeq.mpg

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
Jack Router
Sent: Thursday, April 29, 2010 10:06 PM
To: ccielab_at_groupstudy.com
Subject: 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_item091
86a0
0
800cdfab.shtml
http://www.cisco.com/en/US/tech/tk543/tk545/technologies_tech_note09186a
0080
0
a3a25.shtml

-----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
Blogs and organic groups at http://www.ccie.net

Received on Thu Apr 29 2010 - 23:51:05 ART

This archive was generated by hypermail 2.2.0 : Sat May 01 2010 - 09:49:57 ART