From: Scott Morris (swm@emanon.com)
Date: Thu Jan 05 2006 - 11:59:52 GMT-3
Actually, it's a bit more interesting than that which may yield some
confusion along the way! Now, bear in mind I haven't created a lab with a
bunch of traffic and sniffers to really test this out, but....
While the document on Cisco's web site (and perhaps their implementation?)
states that the first number is a class (1-4) (which corresponds to IP
Precedence 1-4) and then the second number is a drop level, there appears to
be confusion on how that part works.
Cisco's doc says that's a "drop probability" meaning the higher number would
yield a higher probability (e.g. more likely) that the packet would get
dropped. Logically that's odd. As you increase in class numbers, you have
a better chance of getting access to the link during congestion. But as you
increase in drop numbers, you have a better chance of getting dropped within
your class. Weird.
If you look at the RFC 2597, they use some words that are contradictory.
They use "drop probability" which goes along with Cisco's view, but they
also use "drop priority" which would keep the logic similar in that higher
number is better no matter which set we're looking at. The example given in
the RFC in the Appendix really describes this as a "drop precedence" or
"drop priority" which is different.
Being that RFC's aren't particularly binding which is where many people get
into trouble, I'd suggest paying attention to the Cisco docs and just be
aware that it's not working in the manner that you may expect it to! In the
class, higher is better. In the drop probability, lower is better. So the
order of liklihood to die during congestion for Cisco should be:
AF13
AF12
AF11
AF23
AF22
AF21
AF33
AF32
AF31
AF43
AF42
AF41
At least according to that document. Perhaps someone on here as done some
extensive testing in a lab for one of their projects and may be able to shed
better light onto this. At this point, my view is that it's not logical,
but.... Welcome to networking! :)
Scott
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Victor Cappuccio
Sent: Thursday, January 05, 2006 8:51 AM
To: 'Leigh Harrison'; 'FORUM'
Subject: RE: QoS AF priorities
Hello Leigh,
Maybe this like cold help u a little bit more
http://www.cisc.com/en/US/products/sw/iosswrel/ps1834/products_feature_guide
09186a0080080466.html
the AFny PHB defines four AF classes: AF1, AF2, AF3, and AF4. Each class is
assigned a specific amount of buffer space and interface bandwidth,
according to the SLA with the service provider or policy map.
Within each AF class, you can specify three drop precedence (dP) values: 1,
2, and 3.
Assured Forwarding PHB can be expressed as shown in the following example:
AFny
In this example, n represents the AF class number (1, 2, or 3) and y
represents the dP value (1, 2, or 3) within the AFn class.
In instances of network traffic congestion, if packets in a particular AF
class (for example, AF1) need to be dropped, packets in the AF1 class will
be dropped according to the following guideline:
dP(AFny) >= dP(AFnz) >= dP(AFnx)
where dP (AFny) is the probability that packets of the AFny class will be
dropped. In other words, y denotes the dP within an AFn class.
In the following example, packets in the AF13 class will be dropped before
packets in the AF12 class, which in turn will be dropped before packets in
the AF11 class:
dP(AF13) >= dP (AF12) >= dP(AF11)
The dP method penalizes traffic flows within a particular BA that exceed the
assigned bandwidth. Packets on these offending flows could be re-marked by a
policer to a higher drop precedence.
An AFx class can be denoted by the DSCP value, xyzab0, where xyz can be 001,
010, 011, or 100, and ab represents the dP value.
Also I have this formula that could be usefull for anything
8X + 2Y
Af11 == 8(1) + 2(1) = 10
Af41 == 8(4) + 2(1) = 34
-----Mensaje original-----
De: nobody@groupstudy.com [mailto:nobody@groupstudy.com] En nombre de Leigh
Harrison Enviado el: jueves, 05 de enero de 2006 8:57
Para: FORUM
Asunto: QoS AF priorities
All,
I've had a little dig and can't seem to find the answer to my question.
If you have traffic is the following classes:-
af11 af12 af13
af21 af22 af23
af31 af32 af33
af41 af42 af43
I know that af41 is top of the shop and af 13 is at the bottom. My question
is:-
Which order will the packets get dropped off? i.e. will af43 be dropped
before af31?
I'm not sure if they degrade 41, 42, 43, 31, 32....etc, or if they go 41,
31, 21, 11... etc, or any other order.
LH
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