Riddle me this - QoS Bursting vs Physical Interface Bursting

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Content preview: Ok guys, forgive this question if it seems too basic but in
   my QoS studies I seem to find contradiction in how Tc / Bc works for shaping.
   Lets say we have a 10Mbps ethernet link and I have a queue that I will shape
   to 128000 bps and a burst (Bc) set to 1280 bits which makes Tc = 10ms. So
   in other words, this ethernet interface will send packets from the shape
  queue at 1280 bits of traffic for 10ms which to total a speed of 128000 bps.
   Now I also read that this ethernet interface (actually I read that all physical
   interfaces) can only send at either full line rate or not at all...thus the
   contraction is (at least in my mind)....if this 10 Mbps ethernet interface
   were to burst for 10ms (our Tc) we would be sending 100000 bps for every
  Tc and not 1280 bps. [...]

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Date: Fri, 3 Dec 2010 06:46:39 -0800 (PST)
From: Dave Serra <maybeedave_at_yahoo.com>
Subject: Riddle me this - QoS Bursting vs Physical Interface Bursting
To: Cisco certification <ccielab_at_groupstudy.com>
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Ok guys, forgive this question if it seems too basic but in my QoS studies I
seem to find contradiction in how Tc / Bc works for shaping. Lets say we have
a
10Mbps ethernet link and I have a queue that I will shape to 128000 bps and
a
burst (Bc) set to 1280 bits which makes Tc = 10ms. So in other words, this
ethernet interface will send packets from the shape queue at 1280 bits of
traffic for 10ms which to total a speed of 128000 bps. Now I also read that
this ethernet interface (actually I read that all physical interfaces) can
only
send at either full line rate or not at all...thus the contraction is
(at least
in my mind)....if this 10 Mbps ethernet interface were to burst for
10ms (our
Tc) we would be sending 100000 bps for every Tc and not 1280 bps.
So to put this in the form of a question; How is it that a shape queue can
burst
for a 10ms and not send at full line rate? Is it simply a matter of
the
software shapped queue itself only bursting this traffic out of its queue
on to
the mother board bus and to the interface where the tx_ring sends the
traffic at
full line rate of 100000 bps per Tc ?

I'm hoping one of you can
untangle the spagetti I have in my mind

Thanks in advance :)

David Serra

Blogs and organic groups at http://www.ccie.net
Received on Fri Dec 03 2010 - 06:46:39 ART