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Community Member

Rate-limit Burst explanation

Hi All - I need your help to understand the Burst value in the Rate-Limit

Example: rate-limit input access-group 101 20000000 24000 32000

I understand the above configuration limit the traffic to 20Mbps. How to calculate the Burst-Normal (as per example above 24000 Bytes) and Burst-Max(as per above example 32000 Bytes). What is the logic for arriving the Burst-Normal & Burst-Max?

 

Thanks in advance

 

SAIRAM

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Super Bronze

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Posting

The burst sizes, determine the time period the policing is computed over.  Smaller sizes result in smaller measured time periods which more closely mimic the policed rate, but since policing doesn't queue as a real interface would, you're more likely to obtain drops (although you don't get similar real interface queuing delay).

Unless you have need to adjust your burst sizes, you might just allow them to take their default values.

Community Member

Hey Joseph - Thank you for

Hey Joseph - Thank you for the interest shown.

 

if so, Typically whether the Burst size ( burst-normal measured in Bytes during policing) will be higher than the bps value for the below command.

rate-limit bps burst-normal

 

SAIRAM

 

 

Super Bronze

DisclaimerThe Author of this

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The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

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Posting

Unsure there's any "normal".  Cisco, at least for shapers, set Bc so that Tc would be some number of milliseconds (by default). On older equipment, it use to be 25 ms, some of the newer equipment it's 4 ms.

Cisco Employee

 Hi Sairam, below are

 

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

 

 

Community Member

Hi Akash - Thanks for the

Hi Akash - Thanks for the response. I read the explanation and I understood these terminologies like CIR, Bc, Be,Tc are more applicable for Traffic Shaping.


Does this same concept is applicable for Rate-Limiting as well. Thanks again

 

regards / SAIRAM

Super Bronze

DisclaimerThe Author of this

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Yes, it does.  Again, the difference is with policing vs. shaping, the former drops (or remarks) packets, the latter queues packets.

Community Member

Akash - I've literally been

Akash - I've literally been spending all day trying to understand Burst rate within policing. Your explanation hit the right chord with me and now I understand. Thank you!

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