From: Sam.MicroGate@xxxxxxxxxxxxxx
Date: Wed Jun 19 2002 - 09:39:20 GMT-3
Larry
This posting is great. Best explanation I have read for the FRTS. However,
the two formulas
Be + CIR = the depth of the token bucket
Bc = the rate at which tokens are added to the bucke
are troubling me. Meaning the unit for Be is bits and the unit for CIR is
bps. How can you added them. Also the word rate in the second formula means
bps but the unit for Bc is bits only. Am I missing something? Please
clarify. Thanks.
-----Original Message-----
From: Larry Roberts [mailto:larryr@inetbeam.net]
Sent: Wednesday, June 19, 2002 4:36 AM
To: elping; ccielab@groupstudy.com
Subject: Re: CIR, Be, Bc
Shaping mechanisms, such as Frame Relay Traffic Shaping (FRTS) and Committed
Access Rate (CAR) use a token bucket mechanism. The token bucket basically
regulates the amount of data that can be sent during a time period. Data can
only be sent if there are enough free tokens in the token bucket equivalent
to that amount of data. If there are enough tokens, the data is sent and the
appropriate amount of tokens are removed from the Token Bucket. If there is
not enough tokens, the router will buffer the data until the Token Bucket
has enough tokens to send the data and then remove the appropriate amount of
tokens. There are many terms and formulas you must understand to grasp this
concept:
CIR (Committed Information Rate) - This is the average amount of data that
either you (or your service provider) wants you to be able to send
Bc - (Committed Burst) - This is the amount of data that can be sent during
a time interval. There is really no bursting at this point. This is just the
amount of data that you are allowed to send each time interval to meet your
CIR.
Be - (Excess Burst) - Here is where bursting happens. This is normally the
difference between your CIR and actual line access rate (or another
pre-defined parameter by your service provider).
Be + CIR = the depth of the token bucket
Bc = the rate at which tokens are added to the bucket
Now, in order to put all of this together we need one more important piece
of data. That piece of data is the time interval or Tc.
Tc = Bc/CIR
This usually comes out to 0.125 or 125ms. This is the default setting on
Cisco routers.
Example - I have a 128k Frame Relay circuit. I have configured the CIR as
64k. By default, the Tc is 125ms. So, 0.125 times 64,000 = 8,000. Therefore,
my default Bc is 8,000. Therefore, my token bucket fills at this rate and
allows me to send 8k every 125ms. 125ms is one eighth of a second. So, 8,000
times 8 equals 64,000 or 64k per second. Now, my actual line speed is 128k.
I can configure a Be of 64k. This will allow my Token Bucket to fill to a
capacity of 128k. This means that if I don't send any data for a period of
128,000 divided by 8,000 = 16 time intervals, which equals 2 seconds, my
token bucket will fill to capacity and during the next time interval I can
send 128k worth of data or burst to 128k. However, I would then go into a
period of quite time where I couldn't send any data at all until the token
bucket filled back up to CIR, which would be CIR (64,000) divided by Bc
(8,000) = 8 time intervals or one second.
Now, this is probably the perfect example of how to do this right. However,
when you get into higher data rates, the quite time after the burst can get
quite lengthy. Also, a Tc of 125ms works great for data, but this should be
adjusted to 10ms if you are running Voice across the Frame Relay circuit.
HTH,
Larry Roberts
CCIE #7886 (R&S / Security)
----- Original Message -----
From: "elping" <elpingu@acedsl.com>
To: <ccielab@groupstudy.com>
Sent: Tuesday, June 18, 2002 11:43 PM
Subject: CIR, Be, Bc
> Can some one post an explanation of these frame relay
> CIR, Be, Bc
>
> I am a bit confused on the whole thing ...i did not find the doc on
> cisco helpfull
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