[c-nsp] LLQ question

[Rodney Dunn]

If you don't have congestion any traffic can use whatever
bandwidth it wants.

If you have congestion then in the LLQ class it should
guarantee the rate you specify and then any thing above
that is dropped.

We had some inconsistency in our implementations before
if I recall correctly.

On the hardware forwarding platforms you will see it where
you have to do:

priority X
policy Y

for the LLQ class because that is the way the hardware implementation
makes sure the priority class is policed at the rate and doesn't
starve the other classes.

Actually the two statements below are saying the same thing but
in a different manner.

Without congestion LLQ basically doesn't exist.

With congestion, LLQ traffic is guaranteed to get the rate
you configure and anything above that rate during congestion
is dropped.

That's the way I remember it.

Rodney


On Wed, Apr 27, 2005 at 07:29:21AM +1000, Nigel Camp wrote:
> Hi all,
> 
> I have just read a couple of conflicting articles regarding LLQ and was 
> hoping for a definite answer.
> 
> Some sources say that LLQ can't burst past the configured bandwidth / 
> percentage, while others say it can.
> 
> Here are a couple of examples...
> 
>  From http://tinyurl.com/7e89a under the "Guaranteed Bandwidth" heading:
> 
>   Priority traffic metering has the following qualities:
> 
>      * It is much like Committed Access Rate's (CAR) rate limiting, 
> except that priority traffic metering is only performed under 
> congestion conditions. When the device is not congested, the priority 
> class traffic is allowed to exceed its allocated bandwidth. When the 
> device is congested, the priority class traffic above the allocated 
> bandwidth is discarded.
> 
> This contradicts the Cisco press Cisco Qos Exam Certification Guide 
> Second Edition section on LLQ, which states on page 308:
> 
> " The priority command sets the guaranteed minimum bandwidth, which is 
> also the maximum bandwidth! As mentioned earlier, LLQ polices the 
> traffic and in a class that uses the priority command and discards 
> excess traffic.
> 
> It was my understanding that a link not experiencing congestion would 
> allow the burst, but now i'm not so sure.
> 
> Thanks in advance.
> 
> Regards,
> Nigel
> 
> _______________________________________________
> cisco-nsp mailing list  cisco-nsp at puck.nether.net
> https://puck.nether.net/mailman/listinfo/cisco-nsp
> archive at http://puck.nether.net/pipermail/cisco-nsp/