From: Brian McGahan (brian@cyscoexpert.com)
Date: Wed Apr 16 2003 - 23:38:03 GMT-3
Group,
Cisco's definition of CIR does not have anything to do with how
your line is provisioned. CIR in Cisco's terminology simply means your
target rate, or average per second. The MINCIR is what you are
actually
provisioned for by the provider. In the case that the cloud sends your
router a BECN notice, the shaping algorithm will decrease the sending
rate until either the BECN notices stop or you reach the MINCIR. Since
your line is supposed to be guaranteed at MINCIR, it does not make
sense
to drop your sending rate below MINCIR, since this is what is supposed
to be provisioned for you.
The following formulas hold true for Cisco's implementation of
Frame-Relay Traffic Shaping:
Bc = (CIR * Tc)/1000
Be = ((AR - CIR) * Tc)/1000
Bc is bits per interval committed. If you are sending Bc bits
per Tc, you are sending at the CIR.
Be is bits per interval excess. If you are sending Bc + Be bits
per Tc, you are sending at the AR.
You can only send Be bits if you have enough credit built up in
your token bucket. Credit is built up when you do not send enough bits
to equal Bc per interval. Therefore, the purpose of Be is to make up
for unsent Bc traffic, and still sustain the CIR per second. Over an
average, it is not possible to exceed CIR per second, however per
interval it is possible to send at the access rate.
It is also important to note that there is a maximum Tc value
per CIR. The larger your CIR, the smaller your Tc must be. If you
enable traffic shaping, and the values shown in the 'show
traffic-shape'
output do not conform to the above formulas, you have made a
miscalculation.
Delay is also a factor to be taken into account in Frame-Relay
Traffic Shaping. Regardless of what your FRTS values are, the router
always send traffic out an interface based on the serialization of the
line. The serialization is how fast you can actually encapsulate the
traffic on the line and send it. This discussion, however, is beyond
the scope of the CCIE lab. For a more accurate explanation of FRTS and
the token bucket algorithm in general, refer to the Cisco Press book
"Integrating Voice and Data Networks" by Scott Keagy.
http://www.ciscopress.com/catalog/product.asp?product_id={DE856313-09DA-
4A4B-BCC4-93E63D2D4010}
HTH
Brian McGahan, CCIE #8593
Director of Design and Implementation
brian@cyscoexpert.com
CyscoExpert Corporation
Internetwork Consulting & Training
Toll Free: 866.CyscoXP
Fax: 847.674.2625
> > -----Original Message-----
> > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
> Of
> > Jonathan V Hays
> > Sent: Wednesday, April 16, 2003 9:47 AM
> > To: ccielab@groupstudy.com
> > Cc: 'Mike Williams'
> > Subject: RE: MINCIR = CIR?
> >
> > > -----Original Message-----
> > > From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On
> > > Behalf Of Mike Williams
> > > Sent: Wednesday, April 16, 2003 8:45 AM
> > > To: 'Joe Chang'; 'Jeongwoo Park'; ccielab@groupstudy.com
> > > Subject: MINCIR = CIR? (Was: Do you agree with this equation?)
> > >
> > >
> > > I know this sounds silly, but what is the purpose of
> > > MINCIR??!?! Unless everything I've ever learned about Frame
> > > Relay is wrong, CIR is a COMMITTED Information Rate, i.e. the
> > > bandwidth that's guaranteed to you as a customer with no
> > > drops during congestion. So why in the world would you ever
> > > configure a MINCIR that's isn't exacly the same as your CIR?!?!?
> > >
> > > Any input is appreciated.
> > > Mike W.
> >
> > Hi Mike,
> >
> > These are Cisco definitions. Take the trouble to read this document:
> >
> >
>
http://www.cisco.com/en/US/tech/tk713/tk237/technologies_configuration_e
> > xample09186a00800942f8.shtml
> >
> > Basic Cisco definitions are as follows:
> >
> > access rate
> >
> > -Cisco's definition for the physical line speed, or port speed.
> >
> > frame-relay cir
> >
> > -To Cisco, CIR is the average rate defined by the problem statement,
> in
> > bps.
> >
> > -Cisco defines rate given to you by the telco as the "mincir", not
> CIR.
> >
> >
> > -Default cir is 56000 bps.
> >
> > frame-relay mincir
> >
> > -The telco/ISP guaranteed rate.
> >
> > -By default, mincir is equal to half of the cir, in bps
> >
> > mincir = cir/2
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