AF vs CS selectors

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Apr 8th, 2009
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In a book that I'm reading, it gives examples of marking traffic. One of the examples is:


Interactive video AF41 (CoS value 4)

Streaming video CS4 (CoS value 4)


I thought AF and CS were not "better than each other." In other words, I thought they were the same, but it depended on what you did with that class was what mattered. Is that the case, and this book is just showing that AF and CS values can be used, or is there more meaning to this that I'm missing?


Thanks,

John

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Giuseppe Larosa Wed, 04/08/2009 - 09:30
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Hello John,

from a L2 QoS point of view they will be treated in the same way by a device looking at the CoS 802.1p bits, but on a router/switch DSCP aware they can receive different treatment according to an explicit configuration that you have done.


on the switch if you choice to trust dscp the dscp is copied in the internal DSCP used for QoS on multilayer switches.

So there is a potential for a different treatment also on a multilayer switch.


I remember an interesting thread started by you and with good contributes from Edison and Jon in which they provided useful hints.


Hope to help

Giuseppe


John Blakley Wed, 04/08/2009 - 09:32
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Yes, yes, I've started many of this topic :-)


Thanks Giuseppe!

John

Joseph W. Doherty Wed, 04/08/2009 - 10:09
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"I thought AF and CS were not "better than each other." In other words, I thought they were the same, but it depended on what you did with that class was what mattered."


No, they are not the same because they have different (AFxy vs. CSx) DSCP values. They might be treated the same or not.


AF#1 vs. CS# is a bit of a special case, though. If you read the RFCs, CS was really left for backward compatibility with IP Precedence. But with only 4 AF groups, and all the traffic classes defined in RFC 4594, CSx is being used in addition to AFxy.


If they are both mapped into L2 CoS as CoS 4, they can not be distinguished differently.

John Blakley Wed, 04/08/2009 - 10:12
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If they are both mapped into L2 CoS as CoS 4, they can not be distinguished differently.


So they CAN be the same. ;-)


John

Jon Marshall Wed, 04/08/2009 - 10:30
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John


"So they CAN be the same. ;-)"


At layer 2 yes because they are mapped to same CoS value so they will be treated the same way.


At layer 3, well that is up to you really ie. you define how AF41 & CS4 packets are treated. If you wanted you could define the same treatment but you could also define different treatment.


Jon

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