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.
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.
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 18.104.22.168, 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.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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