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Re: QoS Policy Question & Recommendations

Posts: n/a
This is exactly right. Don't try to limit the low class, just guarantee the
other classes get what they need. Extra bandwidth is automatically
distributed amongst the other classes.

This is done using the bandwidth command. I prefer to use percentages. You
should also configure WRED for various reasons.
policy-map whatever
class 1
bandwidth percent 40
class 2
bandwidth percent 20
class 3
bandwidth percent 15

Getting the balance right is a bit of a black art, but this is simple
method I have developed from designing qos policies for large networks.

1) Say your important traffic goes in class1. You need to monitor the amount
and types of drops in this class. Tail drops are bad, random drops should
also be avoided in your class1 (read up WRED for an explanation on these
2) Now attempt to do the same for class2 which is your next in order of
importance (I am avoiding using the word 'priority' as it means something
very specific in qos).
3) Now assign the leftover bandwidth to your class 3. Usuaully you can only
assign 75% of the interface bandwidth in a qos policy, but you can override
this. Examine the amount of tail and random drops you are getting in class
3. If it's too high and this traffic suffers too much when class 2 and
class1 traffic is present, you should scale back the bandwidth assigned to
class 2.
If you start to see random drops in class 2 this is OK. It start to become a
concern when the number of tail drops approaches the number of random drops.

Hope that helps.

"Ignaz Krähenmann" <(E-Mail Removed)> wrote in message
news:4196939a$(E-Mail Removed)...
> Just give each class the share it needs. If you assign a class a certain
> bandwidth, it sure gets it when needed. If the bandwidth is not
> required, it is distributed among the other classes.
> If you have two classes and one should be treated very badly, just
> assign it 1% or so and give the rest to the other class. The low
> priority class can only get 1% as soon as the normal traffic needs the
> bandwidth. Of course, these numbers should be considered more seriously!
> Do not use priority for non-real time traffic. Priority traffic allows
> control of delay, which you should be able to use for real-time
> applications (most typically voice).
> Policing (as in your configuration below) is an option to restrict
> traffic to a certain bandwidth but not to control bandwidth allocation
> of different classes.
> Cisco has pretty good design material on the "Solution Reference Network
> Design" page:
> Michael Love wrote:
> > I'm trying to set up a couple of classes of traffic on our edge router,

> > I want them set up in such a way that as long as nothing else on our

> > needs it, Class "low-priority" gets a portion of our available

> > but that class is the first to give up its bandwidth if any other client
> > needs it.
> >
> > I currently have something like this:
> > ---------------------
> > Class-Map match-any low-priority
> > Match protocol blah1
> > Match protocol blah2
> > Match access-group name blah3
> >
> > Policy Map incoming-test
> > Class low-priority
> > set precedence 0
> > police cir 256000 bc 8000 be 8000
> > conform-action transmit
> > exceed-action drop
> > violate-action drop
> > Class class-default
> > set precedence 2
> > ---------------------
> >
> > This, of course, doesn't do what I want, but it is keeping the traffic

> > "low-priority" from eating up the rest of the network when we are

> > the bandwidth for other things.
> >
> > Let's say our maximum incoming bandwidth is something like 1,000,000bps.
> > Would it work the way I want if I set class-default like this?
> > ---------------------
> > Class class-default
> > police cir 1000000 bc 31250
> > conform-action transmit
> > exceed-action drop
> > ---------------------
> >
> > None of the tweaks I make seem to work the way I want and only impede

> > traffic I want to give the priority. Is this something I need to use
> > random-detect for? I downloaded a number of Cisco docs on QoS like
> > "Configuring a QoS Service Policy", but there aren't enough examples for

> > to get a good idea of how to approach this. Most of them seem, to me, to

> > more along the lines of "policy 1 gets 20%, policy 2 get 30%, and policy

> > gets all the rest". I want something like, "policy 1 gets 25%, as long

> > nothing else on the network needs it"
> >
> > Also, could anyone point me to a good book or online tutorial on QoS

> > design?
> >
> >

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Michael Love
Posts: n/a
I had a flash of insight on this shortly after my post back on the 12th, and
I tried out what you and Ignaz suggested above. Yep, that did it! Once I
took a step back from it and looked at the problem again, this solution was
pretty obvious and looks like it is working fine.

I really wanted to get this taken care of before we get another T1 plugged
into the unit next week.

Thanks to everyone who responded!

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