Hi Sairam,
below are definitions of few terms which are involved here
CIR : committed information rate, in bits per second, defines the rate defined in the traffic contract.
Tc : Time interval,measured in miliseconds, over which the committed burst (Bc) can be sent.
Bc : Committed burst size,measured in bits. This is the amount of traffic that can be sent over the interval Tc.
Be : Excess burst size, in bits. This is number of bits beyond Bc that can sent after a period of inactivity.
Formula to calculate Bc is
Bc = CIR*Tc
Now to understand Bc and Tc, say suppose you have applied 20mbps rate-limit on a 100mbps fastethernet link. Now link can send data (bits) with clock-rate only which is 100mbps so to achieve 20mbps rate on that link router needs to send traffic for 1/5th of a sec and remain idle for 4/5th of a second. 1/5th of sec is 200 msec. If router will send traffic for 200msec and not sending traffic for next 800msec, it can achieve rate of 20mbps but a packet arrived at 199th sec will need to wait for 800msec and this will add unnecessary latency to the packet. To avoid this, router sends few bits for short duration and then does not send for some duration. For the period it sends traffic is called Tc value. and the number of bytes it can send during that interval is called Bc value. So CIR = Bc/Tc (bits per interval).
Now we dont have option to configure Tc but we can configure CIR and Bc, and Tc will automatically be calculated. If we do not configure Bc then router takes default Tc of 125ms and calculates the Bc.
What value to choose for Bc
If we configure Bc too large then Tc will go high for same CIR and this may cause delay or jitter for delay sensitive traffic. For delay sensitive traffic cisco recommends to have Tc 10ms or less.
If i calculate Tc in the given example, it is coming as 9.6ms which is close to 10ms that is why Bc is set as 24000.
Tc = Bc/CIR
= 24000(bytes)/20000000(bits/sec)
= 192000(bits)/20000(bits/ms)
= 192/20
= 9.6 msec
Now Be is to give extra bandwidth for small interval(Tc) to cater some bursty traffic. Assume there is a bucket which gets filled with Bc amount of token in every Tc interval and router can send traffic if there is sufficient amount of token available in the bucket, equal to the packet size. After forwarding packet router reduces same amount of token from the bucket. Size of bucket is also equal to Bc which means if there is no traffic for Tc interval, bucket can not hold more token. Be is to increase the size of bucket to (Bc + Be). Now in every Tc interval bucket will be filled with Bc token and if there is a period of inactivity then in next interval bucket can be filled with extra Bc amount of token till it reaches to (Bc + Be) and if there is any bursty traffic (more than Bc) same can be adjusted. So for a very small period router may send traffic with more rate (higher than CIR, since sending Bc+Be in Tc interval) but over a period does not cross CIR.
You can also use below "Ask the expert" event for QoS to further queries related to QoS.
https://supportforums.cisco.com/discussion/12259571/ask-expert-quality-service-qos-cisco-ios-routers
Please dont forget to rate post if it has been helpful.
Regards,
Akash
