Re: 3560 Policer Burst Value

OK, I have worked this out, at least in my own head to make sense. I
thought I would share with you guys. A lot of this is based on assumptions,
but at least it makes sense to me:

Here is how I see things playing out with an 8000 byte burst size

Assumptions:
Interface is connected at 100Mbps
Policed rate is 3Mbps
Burst Size is set to 8000 Bytes
Ethernet frame size is a maximum 1500 bytes

By this logic, if the host is connected and sending at 100Mbps, and he is
sending 100,000,000 bits per second or 12,500,000 bytes per second. If each
frame is 1500 bytes, it is sending 12,500,000 / 1500 or about 8333 frames
per second. That means every 1/8333 seconds or .00012 seconds a 1500 byte
frame arrives at the switch port and is evaluated against by policer.

When the policer sees the frame, it first adds tokens to the bucket based on
the formula (t1 - t) * policed rate where t1 is the packet arrival time and
t is the last packet arrival time. This is in bytes so... every .00012
seconds a frame arrives and the policer puts 45 bytes into the bucket
(3,000,000/8 * .00012). Obviously, this is a very low number and I think
explains why the 8000 byte limit couldn't handle the bitrate

After not too many intervals of 1500 byte frames coming in at .00012
seconds, the BC bucket would be empty. Now...lets run the math with the BC
size I have chosen, 37,500 bytes. All the other assumptions are the same
here. BC_Size is the size of BC at the beginning of the interval

Time: 0
BC_Size: 37,500

send the 1500 byte frame, leaving BC at 36,000 bytes

Time: .00012 seconds
BC_Size: 36,000 bytes

add 45 bytes to the bucket, send the 1500 byte frame leaving BC at 34,545
bytes

Time: .00024 seconds
BC_Size: 34,545 bytes

add 45 bytes to the bucket, send the 1500 byte frame leaving BC at 33,090
bytes

Time: .00036 seconds
BC_Size: 33,090 bytes

add 45 bytes to the bucket, send the 1500 byte frame leaving BC at 31,635

...

Now, with that math the rate can be sustained for about 28 intervals until
the bucket is emptied and packets would start being dropped. So...that
still doesn't really work because 28 intervals only takes us until .003
seconds until the bucket would be flattened. But then again, my assumptions
could be way off. Everything is based on the idea that the unit is sending
at the negotiated speed of 100Mbps and that is is constantly sending a
stream of fixed length 1500 byte packets. That could be WAY off, but at
least now having really thought through it, I think I have a better
understanding.

If anybody here knowledgeable on the subject happens to read this and think
I'm off my rocker, please enlighten me I'd love to understand deeper.

- Joe

-- 
Regards,
Joe Astorino
CCIE #24347
Blog: http://astorinonetworks.com
"He not busy being born is busy dying" - Dylan
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