What is the correct formula for determining the CIR?
A.
CIR = Bc/Tc
B.
CIR = Bc x Tc
C.
CIR = Tc/Bc
D.
CIR = Bc + Be
E.
CIR = Tc/(Bc+Be)
F.
CIR = (Bc+Be)/Tc
Explanation:
Committed Information Rate (CIR) – the rate the device will send at (on average) over a one second
period.
The default CIR when traffic-shaping is enabled on the interface is 56K. CIR is also referred to as the
“target rate”. Since the device is forced to send at the AR, it does not send all of the time (within one
second) Â in order to send an average amount of data that equals the CIR.
Minimum CIR (mincir) – the rate the service provider guarantees to accept. Theoretically, the
provider will set the DE bit for all traffic above this rate. Mincir is designed to be used in conjunction
with adaptive shaping. With adaptive shaping, the router will throttle down in the event of
congestion. The router will not throttle down below this value.
Committed Burst (Bc) – the number of committed bits allows to be sent during a given interval. The
device sends an average amount of traffic to achieve the CIR. The Bc value defaults to 1/8 of the
configured CIR for speeds below 650K. For speeds above that, it is roughly 1/16 of CIR.
Excess Burst (Be) – the number of non-committed bits the router is allowed to send above Bc during
the first interval (Tc). The amount of Be “credits” is derived from unused Bc credits in previous
intervals. There is no limit to how long Be can “store” unused Bc credits. It is a common
misconception that Be can only store credits from the previous interval or the previous second.
There is no default Be value.
Committed Rate Measurement Interval (Tc) – the time interval over whic Bc or Bc+Be can be
transmitted. The max value is 125 ms and the minimum value is 10 ms.
The Formula
CIR, Tc, and Bc are related mathematically by the following formula:
CIR = Bc/(Tc/1000) Notice the division of Tc by 1000 is used to convert milliseconds into seconds –
the common measurement of CIR and Bc.