Go CCIE Go!! - A Cisco Certified Internetwork Expert

> 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@groupstudy.com [mailto:nobody@groupstudy.com] 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@internetworkexpert.com
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, > > > Blogs and organic groups at http://www.ccie.net > > _______________________________________________________________________ > Subscription information may be found at: > http://www.groupstudy.com/list/CCIELab.html
Blogs and organic groups at http://www.ccie.net