From: Bob Sinclair (bsinclair@netmasterclass.net)
Date: Fri Jan 23 2004 - 11:23:30 GMT-3
Alsontra,
The burst value in policing is designed to work with the TCP windowing
algorithm to help make it possible to actually reach the policed rate.
Remember that whenever a TCP packet gets dropped it will trigger the TCP
slow start algorithm on the sender. This will make the sender back off its
sending rate. If you allowed no burst, you would find that, on average, TCP
traffic will be below the policed rate.
The rules of thumb for these settings are derived from experiment and
statistical analysis. In the real world, you "should" set the burst based
on the specific characteristics of the TCP application(s) you are policing.
HTH,
Bob Sinclair
CCIE #10427, CISSP, MCSE
www.netmasterclass.net
----- Original Message -----
From: <alsontra@hotmail.com>
To: <ccielab@groupstudy.com>
Sent: Thursday, January 22, 2004 1:32 PM
Subject: Policing
> Hello All,
> I'm finding the calculations for CAR based policing a little
> confusing. I've read through the Cisco Policing and Shaping Overview, but
> can't seem to understand normal and extended burst calculations. Cisco
> recommends following values when policing:
>
> normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds
> extended burst = 2 * normal burst
>
> Aside from configured rate, where do the other parameters come from and
what
> are their respective meanings? I understand the classification and marking
> aspects of CAR, so there is no need for any explanation there. However, a
> practical example might help. For instance, lets say I have a 10mbit
> Ethernet link and I've identified some flow that I want to constrain to
56k.
> What would the policy-maps police statement look like?
>
> "police bps burst-normal burst-max conform-action action exceed-action
> action violate-action action"
>
> police 56000 ????? ????? conform-action transmit exceed-action drop
>
> I'm runnung IOS 12.1(11)EA1 on a 3550 and get errors when I put in a
normal
> burst size under 8000bytes.
>
> SW1(config-pmap-c)#police 56000 ?
> <8000-2000000> Normal burst bytes
>
> Also normal and extended burst are in bytes, while rate is in bits???
>
>
> Thanks,
>
> Alsontra
>
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