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RE: Priority command posted 11/30/2008
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> The link mentions that "During congestion conditions, a priority class
cannot use any excess bandwidth". Perhaps that is what you are thinking of.



That's precisely what I was thinking of Paul!  ;-)

 

Now this whole thing gets interesting when you really ponder the definition
of "congestion."  It's tempting to think that 129 kbps of traffic presenting
to an interface clocked at 128 kbps equals/leads to congestion (which it
obviously does) and leave it at that.  But "congestion" actually occurs in
scenarios where you haven't even reached your interface clock rate!  As I
understand things, anyway.  For the purposes of QoS, congestion means that
your TxRing is full and has backed up into the configured SW queue(s).
Depending on TxRing's depth and the burstiness of the traffic, it may well
fill quickly (and recall that the very act of configuring a SW queue on an
interface automatically results in its TxRing being truncated so as to
invoke the SW queue more readily).

 

So I think if you had an interface running at 100 Mbps and you configured a
priority class of 10 Mbps, then presented nothing but the priority class of
traffic to that interface, you'd likely see greater than 10 Mbps of
throughput but less than 100 Mbps, depending on the nature of the traffic
itself (and to an extent I can see there being platform-specific and
interface type-specific differences).  I would expect the SW queue to be
invoked at some point, the priority-class to be policed, and then the SW
queue to be released for a period, so on and so on.

 

(may just be a good lab to put together at some point)

 

  
-----Original Message-----
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of
Anthony Sequeira
Sent: Sunday, November 30, 2008 12:25 PM
To: shank shank
Cc: CCIE Group
Subject: Re: Priority command

Yes - the priority command is used with Low Latency Queuing (LLQ) and
it specifies the amount of priority bandwidth to provide to a type of
traffic (typically Voice). This command also causes a POLICING to the
amount of bandwidth specified.

This is a mechanism to guard against queue starvation for other
traffic forms.

Anthony J. Sequeira, CCIE #15626, CCSI #23251
Senior CCIE Instructor

asequeira@xxxxxxxxxxxxxxxxxxxxxx

Internetwork Expert, Inc.
http://www.InternetworkExpert.com
Toll Free: 877-224-8987
Outside US: 775-826-4344

On Nov 30, 2008, at 1:33 PM, shank shank wrote:

> hello,
>     quick question experts: does the priority command apply a
> maximum limit when specifying a bandwidth? or is it applying the
> minimum bandwidth certain class can get in the policy?
>
> so does this command priority 100 means that the maximum bandwidth
> the class will get is 100k?
>
>
> according  to this link
http://www.ciscosystems.com/application/pdf/paws/10100/priorityvsbw.pdf
>  it does both. can anyone clarify this to me. thanks,
>
>
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