Re: Riddle me this - QoS Bursting vs Physical Interface

Hi David,

There is a confusion you are going into..It is true that the
interfaces can be on(send at full rate) or off (no sensing at all)..Tc
is an interval that of time and Bc is the amount of data that you send
within this Tc..For instance as per your numbers the Bc 1280 will be
sent within a time of Bc divided by physical speed of interface and
not the cir that is Bc/10Mbps..This will be an interval of time
smaller than the Tc..Thus for that Tc you will be sending for that
time you calculated and you will be off for the remaining period
within Tc.

HTH

Karim Jamali

On Dec 3, 2010, at 5:46 PM, Dave Serra <maybeedave_at_yahoo.com> wrote:

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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
>
>
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Received on Fri Dec 03 2010 - 18:42:18 ART