From: Michelle T (mtruman@xxxxxxxxxxxxxxx)
Date: Wed Jan 10 2001 - 12:17:30 GMT-3
Set the CIR to port speed only if you wish to send traffic at an average of
port speed. In other words, use the entire port as much as you can. In most
cases, this is what you want to do. However, there are strong
recommendations from Cisco for Voice over IP to set the CIR to actual CIR to
strictly avoid congestive discard or throttling in the frame relay buffers
within the service provider network.
Adaptive Shaping:
For non-voice, typically, set the CIR to port speed.
Set the Bc to 1/8*CIR (Burst per time interval). For a 128k port Bc=1/8*CIR
=16000 (Note that in any given interval (Tc), no more than 16000 bits may be
transmitted, so... 16000/64000 = insertion delay = 125ms). In all cases, be
sure to calculate the insertion delay required to send the amount of traffic
you have determined you can send to around 125ms or less.
Set the Be=0 in this case since you have already set the CIR to max rate
which is Port speed.
Finally, with adaptive shaping turned on (Becn/Foresight), set the
mincir=fallback rate, which is typically your guaranteed CIR rate.
Strict Shaping: (With some bursting):
128k Port with 64k CIR
CIR=64k (Average data rate)
Bc=1/8*64000=8000
Be=8000 ---> this is the amount of bits to be stored for unused Bc (i.e.
idle time) for transmission when need (so that a 64K cir rate is maintained;
Be= (PIR-CIR)*Tc. In this case: 8000=(128000-64000)*1/8
You would be especially concerned about using adapative shaping at a Hub or
Core router which had High speed links aggregating many remote pvc's at much
slower port speeds. Perhaps you have a T1 port with a pvc to a remote that
has a 56K port and 32K CIR. You might want to use adaptive shaping here so
you don't overload the remote port. If you were doing voice you might wish
to do strict shaping. Finally, You can add priority or custom queuing to the
pvc's for even finer control of what gets sent out first in these valuable
intervals.
Michelle
----- Original Message -----
From: "Shaun Nicholson" <Shaun.Nicholson@kp.org>
To: "ccielab" <ccielab@groupstudy.com>
Sent: Tuesday, January 09, 2001 10:23 AM
Subject: Frame Relay Bc
> OK if I have a frame relay circuit that is running at 128K and I have a
CIR of 64k what would I set the Bc to.
> I know Bc is the outgoing committed burst rate so I would normally set
this to 128K as thats the pipe I'm getting supplied with from my provider.
> In the lab environment would this be correct?
>
> I know in real life thats how I would do it I'm starting to worry about
QOS now as I'm trying to guess what will take over from Appletalk.
>
> Is my assumption correct ?? Bc=128K the size of the pipe I have purchased.
>
> Anyone got any insight to this??
>
> Shaun
>
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