assured forwarding (af) class

Unanswered Question
Apr 21st, 2008
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Are there any priority among AF class?

I knew about drop precedence within a class.

Hey, sometime asking here got quicker answer than googling. ;)


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bmcginn Mon, 04/21/2008 - 22:06
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Hi there,

Is there any priority among the AF class?

It depends; the AF class encompasses several levels of precedence.. eg Af11, af12, af13, af21, af22, af23, af31, af32, af33, af41, af42, af43. Each AF class uses the six dscp bits, the first three of which are the IP Precedence bits. The last three bits can be used as drop precedence for RED/WRED..

So yes, there is a basic priority within an AF class. AF41 has a higher priority than AF31, AF21 or AF11 in a normal sense. You can in fact configure the qos to have af41, cs4, ipp4 have less priority than af11, cs1 or ipp1.

By and large the higher the AF class number the higher the priority.

Working from top to bottom:

IPP5 CS5 EF voice

IPP4 CS4 AF41,AF42,AF43 video

IPP3 CS3 AF31,AF32,AF33 Mission Critical

IPP2 CS2 AF21,AF22,AF23 Transactional

IPP1 CS1 AF11,AF12,AF13 Bulk Data

IPP0 CS0 BE Scavenger

cisco_lad2004 Mon, 04/21/2008 - 22:07
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AFxy means AFx1 has a better dropping rate than AFx2 which is better than AFx3



mohammedmahmoud Tue, 04/22/2008 - 00:29
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AFxy translates to the following; x implies one of four queues (values 1 through 4), and y implies one of three drop priorities (values 1 through 3). The higher the x the higher the Queuing treatment as per the DiffServ model, and the higher the y the higher the drop priority accordingly AF41 is the best and AF13 is the worst.


Mohammed Mahmoud.

smallrain_2 Tue, 04/22/2008 - 11:03
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I still want to verify as I thought I was given a bit different answers.

I do understand drop precedence, value of y in AFxy. But what standard says about class itself. I am talking about x here.

Some said higher x has higher queuing priority. I want to know this is a "common practise" or a "standard".

Thanks again,

cisco_lad2004 Tue, 04/22/2008 - 12:22
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higher x is better Queuing.

higher y is more dropping in case of congestion, so its higher probability drop.

AF41 will be treated better than AF31

AF43 will drop more than AF41

AF22 will drop more than AF32 (even if packets are treated better).

AF46 is usually applied for VoIP so Low Latency.

X is ur queuing treatment, y is ur drop probability.



Joseph W. Doherty Tue, 04/22/2008 - 13:15
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With regard to standards, the x in AFxy maps one of the AF classes into a Class Selector Codepoint, see RFC 2474, which defines the relationship between different Class Selectors. Section, has ". . . PHBs selected by a Class Selector Codepoint SHOULD give packets a probability of timely forwarding that is not lower than that given to packets marked with a Class Selector codepoint of lower relative order, under reasonable operating conditions and traffic loads." Because of the "SHOULD" there is no requirement for different treatment between CS 1..4.

With regard to "common practice", either DSCP is totally ignored or it might be treated with bias in line with the prior IP Precedence recommendations, i.e. higher is better but how "better" can vary.


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