From: Frank B (frank@xxxxxxxxxxxx)
Date: Tue Aug 13 2002 - 05:30:38 GMT-3
Hey folks...sorry about the re-hash but after receiving my last post I
reread it and realized a couple of SNAFUs within.
My statement: "Bc is commonly set to 1/8 of the CIR" is not exactly
correct. CIR is a rate of bits per second, Bc is in bits making Tc just
a time period in seconds (basic algebra-the bits canx each other out and
there's only seconds remaining...right?) well, you get it.
My 2nd foul-up: I found that Be is set to zero bits by default (not
sure what I was thinking)...making Bc the max you can transmit in a
given time interval by default. I should think things through before I
transmit.
Guess I don't get partial credit huh? Later, Frank
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Frank B
Sent: Monday, August 12, 2002 9:38 PM
To: 'Mark Vann'; ccielab@groupstudy.com
Cc: 'Andri Bersvendsen'
Subject: RE: FRTS
Mark,
The way I attempt to keep it straight in my warped mind is that
what you're paying the carrier for is the "minimum" you ever want to
transmit at--else you're being ripped off ;-> So, the rate they give
you (whether they call CIR or whatever) should be configured on a Cisco
router as equal the minCIR. The CIR (in Cisco-eese) is the rate you
want to transmit at under uncongested conditions--usually set to the
access rate (aka port speed) and of course this can't possibly be faster
than the access rate of the interface.
Tc=Bc/CIR --this is usually 125ms because Bc is commonly set to 1/8 of
the CIR...but doesn't have to be!
BTW....that's also why there's usually 8 time periods per second. Which
brings us to Be (which you didn't ask about but we're so close)
Be is the amount you can transmit over and above the Bc within that
first time period of each second...IF you have credit built up (not to
exceed the volume of the infamous and invisible token bucket right?) I
beleve that by default Be=Bc which in that case gives you the limit of
Bc in each time period within each second--not 100% sure about that one
though.
Now I hope you don't mind a question on the topic from me...where in an
"official" Cisco document does it state that upon receipt of a BECN
within a given time interval that the transmit rate is decreased by 25
percent? I've seen it around (and on page 385 of Solie's book for
example) but is this in fact correct? And in any event, is this
configurable?...and the obvious follow-on "How do you do it?"
Thanks and aloha man! Frank
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Andri Bersvendsen
Sent: Sunday, August 11, 2002 10:42 PM
To: Mark Vann; ccielab@groupstudy.com
Subject: Re: FRTS
Bc = 1/8 of CIR only if Tc=125mS (this is the normal/default value for
Tc).
Bc = CIR*Tc
Example:
Bc=?
CIR=64000
Tc=0.125
Bc=64000*0.125
Bc=8000
Bc=8Kbps
CIR do not have anything with port speed (also called access rate (AR))
to do. It must be lower than the access rate.
And now LAB14:
On page 382 the following information is given:
port speed (JPL)=1.544 Mbps
CIR=32Kbps.
port speed (nasa_houston)=64Kbps
In FRTS you must configure MinCIR=32Kbps.
And Bc is calculated from CIR in FRTS.
>Can someone please clarify something for me? When
>cisco says that bc = 1/8 of cir, that basically means
>that 8/CIR=Bc . If this is so are they refering to CIR
>being the provider CIR or the CIR that is used in TS ,
>such as Port speed? I am just trying to figure out how
>Karl Solie arrived at this values on lab 14 in the
>preparation volume 1.
>Thanks
>
>
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