From: Petr Lapukhov (petr@internetworkexpert.com)
Date: Fri Jul 14 2006 - 01:48:22 ART
Patricia,
Average bit rate with policer is a bit tricky thing :) As you
understand, every packet leave policer at wire speed (actual
interface rate). Hence, to introduce an average bitrate, one
need to introduce some "averaging" scheme. This is where
burst comes to play:
By definition of policer, if you observe policer's output
for the duration of time [Tc=Bc/AvgBitrate], you will collect
no more then Bc bytes of *conforming* (marked as conforming)
packets. Tc place role of averaging interval here..
Policer's output:
PPPPPP----PPPPPPP---PPPP---
|_______Bc_________|
T0-----------------------------T1 : Time
T1-T0=Tc, Avg = Bc/Tc.
If you set bigger burst, you "extend" you averaging interval,
hence permitting more "spikes" of packets flow. The ultimate
goal of using burst is to adapt to "unstable" flows, like
that produces by many users surfing web. You may usually
find "dense" and "sparse" blocks of packets within that flows.
You see, if you police "uniform" flow, like stable ping from cisco
router, it does no matter what is your burst size. You will always
get that same average bitrate over any output burst.
PPPPPPPPPPPPPPPPPPPPP ->
P-P-P-P-P-P-P-P-P-P-P-P-P-P
But if you have flow like that
PPPP--------PPPPPPPP--------PPPPP
You would certainly benefit from setting bigger burst size,
because that permits you to accumulate some *credit*
when your flow is silent, and use the credit when a "spike"
arrives.
Something like that:
PPPP--------PPPPPPPP--------PPPPP is policed to
(with small burst) ->
P-P-P---------P-P-P-P-P-P-------P-P-P-P
OR with large burst: ->
PPPP--------PPPPP----------------PPPPP
Note that average bit rate remains the same, it is just
measured over different bursts (or different averaging intervals)
HTH
-- Petr Lapukhov, CCIE #16379 petr@internetworkexpert.comInternetwork Expert, Inc. http://www.InternetworkExpert.com Toll Free: 877-224-8987 Outside US: 775-826-4344
2006/7/14, Patricia Loreal <ploreal@gmail.com>: > > Hello Dears. > > What is the relation between average bit rate and the BC and BE values > when > we configure sr-TCM or tr-TCM? > > Say: > > Average Bit Rage = 192000 bps > Bc = 3000 or (KB) CIR in tr-TCM > Be = 5000 or (KB) PIR in tr-TCM > > I understand that when a packet arrives with a determined size it will be > discounted from the Bc bucket if tokens are available and if not It would > check the Be Token Bucket. (be/bc are in bytes) > > OK, that's the meter part that could help me decide what to do with the > packet that Conforms or exceed the values (remark, drop, transmit) / > Violate > in tr-TCM > > But what is the relation with the Average bit rate? > > What the process exactly when a packet arrives, in relation to the Average > Bit Rate ( that is in bits/sec) > > > Say: > > P1, 300 Bytes, mSec = 0 > P2, 500 Bytes, mSec = 25 > P3, 800 Bytes, mSec = 400 > P4, 1000 Bytes, mSec = 2 > P5, 130 Bytes, mSec = 40 > > Thanks for you kindest help > Patricia Loreal > > _______________________________________________________________________ > Subscription information may be found at: > http://www.groupstudy.com/list/CCIELab.html
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