From: Brian McGahan (bmcgahan@internetworkexpert.com)
Date: Thu Jun 08 2006 - 10:44:40 ART
The term "Bc" means "Burst Committed", i.e. how much am I allocated to
send on a per interval basis. The term "Be" means "Burst Excess", i.e.
how much above my normal allotment can I send if either I have built up
credit (shape average) or if I want to pay back a debt later (shape
peak, policing).
Brian McGahan, CCIE #8593
bmcgahan@internetworkexpert.com
Internetwork Expert, Inc.
http://www.InternetworkExpert.com
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> -----Original Message-----
> From: Godswill Oletu [mailto:oletu@inbox.lv]
> Sent: Thursday, June 08, 2006 7:08 AM
> To: Navin MS; Roberto Fernandez; Brian McGahan; Leigh Harrison; Andi
> Bennett
> Cc: ccielab@groupstudy.com
> Subject: Re: FRTS Lab 1 - 7.2 question.
>
> Navin,
>
> We might be saying the same but differently; the thing is I really do
not
> consider it a burst, if you allow me to burst upto Bc within each Tc
but
> at
> the end of the day, I cannot send more than my CIR. After all is said
and
> done, I am still limited to my CIR.
>
> And, I am kind of having a weird feeling that, if a question states
that
> the
> CIR is eg 128kbps and the question further states that allow for some
> burst
> and one configure only CIR and Bc; he/she have not sufficiently
fulfill
> the
> tasks set forth in the question.
>
> Configuring CIR and Bc only fulfill the aspect of the task that is
dealing
> with CIR and by including a Bc; you are telling the router that, you
want
> to
> be in charge; you want to be the one who decides the rate at which you
> want
> to send on each Tc timeslot and by that you are also dictating to the
> router
> the value of the Tc to be used. And the router will only accept you Bc
> value
> if it makes sense. If on the other hand, you did not configure Bc, the
> router will configure one for you, but all these have not fulfill the
> aspect
> of the task that states '....allow for some burst'.
>
> R3640-R1(config-pmap-c)#shape average 128000 ?
> <256-154400000> bits per interval, sustained. Needs to be multiple
of
> 128.
> Recommend not to configure it, the algorithm will
find
> out
> the best value
> <cr>
>
> R3640-R1(config-pmap-c)#shape average 128000
>
> *note, what the router is saying about Bc'...recommended not to
configure
> it, the algorithm will find out the best value'
>
> >
> > 1) Send traffic at a constant rate of CIR (in which case Bc of 4000
is
> unused). Though you were
> > allowed to burst, you didn't :)
> >
>
> Can you explain this further? Is it possible not to use Bc? How can
one
> send
> at constant rate of CIR without using Bc? My experience had been, even
if
> you did not specify Bc, the router will calculate and impose one on
you.
>
> The only theorical time, I think one can send at constant rate of CIR
will
> be when Tc= 1 sec, but the maximum recommended Tc is 10ms, so whether,
we
> like it or not, we will be forced to divide our CIR into junks of Bc
and
> are
> allowed to burst from 0 to that Bc value within each Tc.
>
> Look at my attempt at my failed attempt at forcing my router to accept
a
> very high Tc of 2ms; which is a long shot from the theoritical Tc
vlaue of
> 1sec assumed above.
>
> R3640-R1(config-pmap-c)#shape average 128000 256
> less than 1000 bits in an interval doesn't make sense
>
> R3640-R1(config-pmap-c)#
>
> The router took on some little attitude there, telling me that what I
am
> trying to accomplish 'doen't make sense'.
>
> >
> > 3) Send a burst of (Bc + Be) bits per Time interval (if Be is
> configured,
> in which case you are
> > using Bc of 4000 and Be of 2000 say). When you do this, you are
> effectively sending more than CIR.
> > That is, your Tx rate = (Bc+Be)/Tc which is greater than (Bc/Tc).
> >
>
> This not guarantee at all time, in essence your effective/constant Tx
rate
> will not be (Bc+Be)/Tc; In shaping with Bc and Be configured, you can
only
> send above Bc per timing interval when you have accumulated credit to
do
> so
> from the previous timing interval.
>
> Read the section 'Traffic Sahping and Rate of Transfer here:
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
fq
> os_c/fqcprt4/qcfpolsh.htm
>
> In policing, configuring Bc & Be and the process of determining how
the Be
> get used, involve a complex set of algorithms, that will include
borrowing
> tokens, compounded debts, etc.
>
> Read the section 'Rate Limits' and other sections below it:
>
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/
fq
> os_c/fqcprt4/qcfpolsh.htm.
>
> Excuse, my rather lengthy email and also remember that, there are many
QoS
> gurus on this list and I am not one of them.
>
> HTH
> Godswill Oletu
> HTH
> Godswill Oletu
>
>
> ----- Original Message -----
> From: "Navin MS" <navin_ms07@yahoo.com>
> To: "Godswill Oletu" <oletu@inbox.lv>; "Roberto Fernandez"
> <rofernandez@us.telefonica.com>; "Brian McGahan"
> <bmcgahan@internetworkexpert.com>; "Leigh Harrison"
<ccileigh@gmail.com>;
> "Andi Bennett" <bigandibennett@yahoo.com>
> Cc: <ccielab@groupstudy.com>
> Sent: Thursday, June 08, 2006 12:16 AM
> Subject: Re: FRTS Lab 1 - 7.2 question.
>
>
> > Yes, When it comes to plain English.. a burst is thought of
something in
> excess that a router is
> > sending into the FR cloud. BUT from a frame-relay perspective, it
simply
> says how many bits in
> > burst can be sent in one Time interval. Bc just gives us bursting
> capability which is very much
> > required given the sporadic nature of real-time traffic. I have an
easy
> alternate way of
> > understanding this. One can look at it this way.
> >
> > For a customer there are 4 options...
> >
> > 1) Send traffic at a constant rate of CIR (in which case Bc of 4000
is
> unused). Though you were
> > allowed to burst, you didn't :)
> >
> > 2) Send a burst of Bc bits per Time interval (in which case you are
> using
> Bc of 4000 bits). When
> > you do this, you are effectively sending CIR bps (Bc/Tc) as agreed.
> >
> > 3) Send a burst of (Bc + Be) bits per Time interval (if Be is
> configured,
> in which case you are
> > using Bc of 4000 and Be of 2000 say). When you do this, you are
> effectively sending more than CIR.
> > That is, your Tx rate = (Bc+Be)/Tc which is greater than (Bc/Tc).
> >
> > 4) Send traffic which is constant for some part of the Time interval
and
> random burst for some
> > other part of Time interval. Eg: For a period of say, Tc/2 you sent
3000
> bits at a constant
> > bit-rate and another Tc/2 you send a random burst of 1000 bits.
> Together,
> you sent 4000 bits in
> > that interval. THAT CONFORMS TO CIR. Now, if you send anything more
than
> 4000 bits, you will be
> > using the extended Burst capability (Be).
> >
> > Similar reasoning goes for policing too.
> >
> > Just my 2 cents.
> > Naveen.
> >
> >
> >
> > --- Godswill Oletu <oletu@inbox.lv> wrote:
> >
> > > Roberto,
> > >
> > > Though, I have not checked the question to see if it is shaping or
> policing,
> > > but if we assume that it is policing as you have just said, I am
lost
> at
> > > how:
> > >
> > > Police cir 128000 bc 4000
> > >
> > > Will result in a burst, as the original poster indicated. The
command
> above
> > > is just telling the router that, in order not to create a
scheduling
> > > concern, please kindly send 4000 bytes per burst or timing
interval
> (Tc).
> > >
> > > Bc is committed burst, you have paid for 128Kbps and you are
telling
> the
> > > router to allow you to burst your traffic from 0 to 4000 per
timing
> interval
> > > (Tc), and this commitment is based on your CIR. The concept of
burst
> as
> we
> > > understand it in everyday English only start coming into play with
Be
> or
> > > excess burst.
> > >
> > > Also, if you look at the computation you just did; you are
equating
> CIR
> to
> > > be the same the same value as Bc, but using different metrics,
which
> might
> > > not be the correct thing to do.
> > >
> > > The CIR given was 128kbps, convert this to bytes will be be 128000
/ 8
> =
> > > 16000 bytes and you divided that by 4 to get 1/4Bc (4000 bytes)
that
> was
> > > asked for.
> > >
> > > Let assume that a full Bc was asked for instead of 1/4Bc, you see
> that
> your
> > > computation will make Bc = 16000 bytes and this is the same amount
of
> > > traffic our CIR is allowing us to transmit per second. In a one
second
> > > traffic snapshot, the difference between 128kbps and 16000 bytes
is
> the
> same
> > > as the difference between $1 and 100 cents.
> > >
> > > As you can see, that might not be the correct formula to computer
Bc
> either
> > > in policing or in shaping.
> > >
> > > HTH
> > > Godswill Oletu
> > >
> > >
> > > ----- Original Message -----
> > > From: "Roberto Fernandez" <rofernandez@us.telefonica.com>
> > > To: "Brian McGahan" <bmcgahan@internetworkexpert.com>; "Godswill
> Oletu"
> > > <oletu@inbox.lv>; "Leigh Harrison" <ccileigh@gmail.com>; "Andi
> Bennett"
> > > <bigandibennett@yahoo.com>
> > > Cc: <ccielab@groupstudy.com>
> > > Sent: Wednesday, June 07, 2006 2:21 PM
> > > Subject: RE: FRTS Lab 1 - 7.2 question.
> > >
> > >
> > > Friends,
> > >
> > > Well... checking into 7.2 question of Version 3 WB, it is not a
> question
> > > about FRTS it is about policing.
> > >
> > > With FRTS Bc is bits, but with policing is Bytes, thus 128 kbits/4
=
> 32
> > > kbits => 4K Bytes
> > >
> > > The answer says
> > >
> > > Police cir 128000 bc 4000
> > >
> > >
> > >
> > > Best Regards,
> > > Roberto
> > >
> > > -----Original Message-----
> > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
Behalf
> Of
> > > Brian McGahan
> > > Sent: Tuesday, June 06, 2006 9:45 AM
> > > To: Godswill Oletu; Leigh Harrison; Andi Bennett
> > > Cc: ccielab@groupstudy.com
> > > Subject: RE: FRTS Lab 1 - 7.2 question.
> > >
> > > > I think it has to do the value of the default 'Tc' (the Tc value
> used
> > > when
> > > > none is supplied) which is 125ms.
> > >
> > > In this case yes, but not always. The default Tc is a function
> > > of the CIR. The higher the CIR the lower the maximum (and
default)
> Tc.
> > > Change the "frame-relay cir" value in a map-class with no other
values
> > > configured and look at the "show traffic-shape" output. As the
CIR
> goes
> > > up you will see the Tc go down.
> > >
> > >
> > > HTH,
> > >
> > > Brian McGahan, CCIE #8593
> > > bmcgahan@internetworkexpert.com
> > >
> > > Internetwork Expert, Inc.
> > > http://www.InternetworkExpert.com
> > > Toll Free: 877-224-8987 x 705
> > > Outside US: 775-826-4344 x 705
> > > 24/7 Support: http://forum.internetworkexpert.com
> > > Live Chat: http://www.internetworkexpert.com/chat/
> > >
> > >
> > > > -----Original Message-----
> > > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
Behalf
> > > Of
> > > > Godswill Oletu
> > > > Sent: Tuesday, June 06, 2006 7:23 AM
> > > > To: Leigh Harrison; Andi Bennett
> > > > Cc: ccielab@groupstudy.com
> > > > Subject: Re: FRTS Lab 1 - 7.2 question.
> > > >
> > > > LH,
> > > >
> > > > Maybe, you are talking about 'fragments' which are in bytes,
> > > otherwise,
> > > > 'CIR' are in bits/second; 'Bc' in Bits and 'Be' in Bits.
> > > >
> > > > I think it has to do the value of the default 'Tc' (the Tc value
> used
> > > when
> > > > none is supplied) which is 125ms.
> > > >
> > > > Be=(AR- CIR) * Tc/1000
> > > > =(128000 * 125/1000
> > > > =16000
> > > >
> > > > 1/4 Be = Be / 4 = 16000/ 4
> > > > = 4000
> > > >
> > > > My only problem is that, the orignal posted mentioned 'Bc'
instead
> of
> > > > 'Be'.
> > > > I do not see how 'Bc' will result in a Burst, except I am
missing
> > > > something
> > > > here or in the question. 'Bc' simply tell us how much need to be
> > > > transmitted
> > > > per timing interval to conform to our 'CIR'; which is in our
case
> > > every
> > > > 125ms and to conform to our 'CIR' of 128K, that value is 16000.
'Be'
> > > tell
> > > > us
> > > > that, after fulfilling our allocated quota and we have room, how
> much
> > > > above
> > > > CIR should we Burst?
> > > >
> > > > Since, the Access Rate (AR) was not given in the task, it is
safe to
> > > > assume
> > > > that 'AR' is equal to 'CIR', making both 'Bc' & 'Be' to be equal
and
> > > 1/4
> > > > of
> > > > Be will be 4000.
> > > >
> > > > Please, set me straight if I am missing something here.
> > > >
> > > > Thanks.
> > > > Godswill Oletu
> > > >
> > > >
> > > > ----- Original Message -----
> > > > From: "Leigh Harrison" <ccileigh@gmail.com>
> > > > To: "Andi Bennett" <bigandibennett@yahoo.com>
> > > > Cc: <ccielab@groupstudy.com>
> > > > Sent: Tuesday, June 06, 2006 6:39 AM
> > > > Subject: Re: FRTS Lab 1 - 7.2 question.
> > > >
> > > >
> > > > > Hey there Andi,
> > > > >
> > > > > CIR is in bits, Bc is in bytes
> > > > >
> > > > > 128,000 / 4 = 32,000 _bits_ per interval
> > > > >
> > > > > 32,000 / 8 = 4,000 _bytes_ per interval
> > > > >
> > > > > Hope that helps a bit,
> > > > >
> > > > > LH
> > > > >
> > > > > Andi Bennett wrote:
> > > > > > Dear group.
> > > > > > Being a bit crap at FRTS can somebody explain the answer
to
> the
> > > Lab
> > > > 1
> > > > 7.2 question.
> > > > > >
> > > > > > 128K CIR and allow for 1/4th for burst.
> > > > > >
> > > > > > How does this come up with bc = 4000 ?
> > > > > >
> > > > > > Am I missing something here?
> > > > > > Cheers
> > > > > > A.
> > > > > >
> > > > > >
> > > > > >
> > > > > > Send instant messages to your online friends
> > > > http://uk.messenger.yahoo.com
> > > > > >
> > > > > >
> > > >
> > >
>
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