From: R.J.Neill Craven (ncraven@xxxxxxxxxxxxxxx)
Date: Wed Nov 14 2001 - 14:00:38 GMT-3
Wade,
I have very different view as to how CAR works! The first number
(1544000) is indeed the bit rate, but the remaining two values are
NOT in addition to the first. The second, the burst (in bytes) and
sometimes referred to as Bc, helps to indicate the size of the token
bucket and is used to determine the time over which the traffic is
rate limited (Tc). In particular, Tc=Bc*8/CIR. The "*8" is used to
convert the bytes of Bc to bits.
Tc is a very important parameter in rate limiting or traffic shaping.
Suppose as a new CCIE you are offered a salary of $120,000 per year
but you are offered several payment strategies. You could take one
payment of $120,000, or 12 payments of $10,000, or 24 payments of
$5,000, or .... I have never been very good at budgeting money so I
would choose to spread the salary over more periods. In other words,
I want the salary to arrive at a fairly even rate. Someone else might
prefer the lump sum approach. Any way you look at it though, we are
only getting $120,000 per year, each! In the context of CAR (and
disregarding the bits and bytes business) I would define the rate
limit for my salary as "rate-limit input $120000 $5000 $5000". Tc
would be 5000/120000 or 1/24th of a year giving me 24 payments of
$5000 each.
I think you will agree that having some way to define the time over
which the rate is limited is particularly useful. We just need to be
clever about how we choose the Bc to achieve the right Tc.
The third parameter (typically called Be), is the excess burst and in
the context of of my pay, indicates the maximum I can be paid during
each pay cycle. Perhaps I have an overtime provision in my contract
which allows me to accumulate additional pay credits. But the company
can limit their exposure by indicating that they will not pay more
than a certain total amount during each period. And they will not let
me work overtime continuously, either. So if I try to inflate my pay
package by working a lot of overtime, they are going to stop me
working overtime, period! Suppose I am allowed to accumulate $1,000
in overtime pay in addition to my regular salary, I would define this
as "rate-limit input $120000 $5000 $6000".
So, Kevin, to get your desired effect, I would have used "rate-limit
input 154000 6000 8000" or something similar to allow for bursts of
1/3rd of the average rate. (Adding 1/3rd to 6000 bytes makes 8000
bytes; 8000 bytes during the same period increases the rate by 1/3rd
or approximately the difference between 1.544 Mbps and 2.048 Mbps.)
You could use bigger values than 6000 and 8000, so long as you keep
the ratio of 3 to 4.
Cheers,
Neill
At 9:33 AM -0600 14/11/01, Wade Edwards wrote:
>The rate-limit command you specified is going to allow 1.544Mb normal
>traffic then allow another 2.048Mb of burst traffic then it will drop
>all traffic after 3.592Mb. The numbers should be 1544000 63000 63000.
>This will give you 1.544Mb normal traffic then allow another .504Mb
>(63000 bytes x 8 gives you 504000 bits or .504Mb) for a total of 2.048Mb
>of traffic then all traffic will be dropped.
>
>The burst numbers are in addition to the normal traffic and the second
>and third numbers are in bytes and the first number is in bits. Hope
>this helps.
>
>L8r.
>
>-----Original Message-----
>From: tom cheung [mailto:tkc9789@hotmail.com]
>Sent: Tuesday, November 13, 2001 8:03 PM
>To: kevin@btamail.net.cn; ccielab@groupstudy.com
>Subject: Re: How to calculate CAR?
>
>
>rate-limit in 1544000 256000 256000 conform transmit exceed drop
>
>
>>From: Kevin <kevin@btamail.net.cn>
>>Reply-To: Kevin <kevin@btamail.net.cn>
>>To: "ccielab@groupstudy.com" <ccielab@groupstudy.com>
>>Subject: How to calculate CAR?
>>Date: Wed, 14 Nov 2001 9:18:52 +0800
>>
>>Hello all, I do learn Commit Access Rate(CAR).
>>I found there are three parameter about car.
>>If I want to give one my custom a contract bandwidth on FastEthernet
>which
>>commit access rate 1.544M and burst size is 2.048M. How to setup the
>second
>>and third parameter on "rate-limit" command. Of course the first
>parameter
>>is 1544000, but which value is precision for normal burst size and max
>>burst size.
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