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Cisco QOS - Policing

Posted on 2010-08-22
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Hi All,

Need your help to understand what is the Token Bucket Concept? I dont understand what it really means by new tokens are added into the token bucket based on the interpacket arrival rate and the CIR. Every time a packet is policed, new tokens are added back into the token bucket.  

In the below case, is my TOken Bucket size 884000? Can you please help me to understand what really cause the exceed action to drop kick in?

police 884000 8000 conform-action set-dscp-transmit ef exceed-action drop
Thanks
Hari
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Question by:kamalgopi
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Alex Bahar earned 500 total points
ID: 33499988
You're not alone. People often get confused with policing calculations.
Your bucket size is 8000 bytes. This means you are allowed to send 8000 bytes from your bucket within Tc period.
To simplify assume you receive packets every 5 msec (0.005 sec). You add tokens at the rate of (0.005 * 884000) = 4420 bits in 0.005 sec = 552 bytes. That means you are allowed to add 552 bytes to your bucket every 0.005 seconds.
Assume you started with your bucket full. You have 8000 bytes in it. 0.005 seconds later, you received 5000 bytes in a packet. This packet conforms because you have 8000 bytes in your bucket. So set-dscp-transmit is executed. 8000-5000=3000 bytes remain in your bucket. Referring previous calculation, you add 552 bytes to your token in 0.005 secs later. Your bucket has 3552 bytes in it. Now another packet arrives with 4000 bytes in it. This is more than the bytes in your bucket (3352). So you execute exceed action drop the packet. Referring previous calculation, you add 552 bytes to your token in 0.005 secs later. Your bucket now has 4114 bytes in it. Now another packet arrives with 4000 bytes in it.  This packet conforms because you have 4114 bytes in your bucket. So set-dscp-transmit is executed. 4114-4000=114 bytes remain in your bucket. Then there is a silence of 0.5 second period you did not receive any packets. This means 552*100=5520 bytes get added to your bucket.
I hope this helps.
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by:kamalgopi
ID: 33508093
Fantastic. Now i have some understanding on how it works. Now i need to go and look for what is the best figure for a Bucket for a 1M Link.
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