Re: 3560 Policer Burst Value

Joe,

try 250000

On Thu, Sep 15, 2011 at 3:31 PM, Joe Astorino <joeastorino1982_at_gmail.com>wrote:

> I am having a real hard time finding good information on this topic for use
> in the real world. In the lab, we would usually just configure the burst
> size we are told on a Cat 3560. I have done a LOT of reading on it, and
> there are a lot of conflicting stories with regards to this.
>
> Basically, I am trying to find out how to calculate an optimal burst value
> on a 3560 QoS policy doing policing. As you probably know the syntax looks
> like this:
>
> police [rate in bits/s] [burst size in bytes]. Remember, this is policing
> not shaping so the classic shaping formula of tc = bc/cir has no relevance
> here mainly because the token refresh rate is not based on a static set
> amount of time. The burst size is actually the size of the token bucket
> itself in bytes, not a rate of any kind and it is filled as a function of
> the policed rate and the packet arrival rate. The refill rate of the bucket
> is not based on a static amount of time like in FRTS for example. It
> basically says "how long was it since the last packet...multiply that times
> the policed rate, and divide by 8 to give me bytes". In other words it
> pro-rates the tokens. Makes sense.
>
> Anyways...I have found 2 sort of "methods" to calculating this, but they
> are
> so far off from one another I am not quite sure which one to use in the
> real
> world.
>
> Method 1: The classic CAR formula we see on routers: (rate * 1.5) / 8.
> This basically gives you 1.5x the policed rate, and converts it to bytes.
> Makes sense.
> Method 2: 2x the amount of traffic sent during a single RTT.
>
> In my case, I am trying to police a video conferencing endpoint to 3Mbps so
> by method 1 that gives me a burst size of 562,500 bytes. Using method 2,
> let's just say I have an average RTT of 100ms. That method would yield a
> burst size of 75,000 bytes. That is a HUGE difference
>
> This came about because the video endpoint was dropping frames. I noticed
> the policed rate in the policy was 3,000,000 but the burst size was 8000
> bytes (the lowest possible value). When I changed the burst based on a
> 100ms RTT and the above formula the problem went away, but now I am having
> doubts on the proper value to use here.
>
> Does anybody have any insight on how to actually calculate this properly?
>
> --
> Regards,
>
> Joe Astorino
> CCIE #24347
> Blog: http://astorinonetworks.com
>
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