QoS: Class Selector PHB and DSCP Values

Since the DSCP field overlaps with the previously defined IPP field within the ToS byte, RFC 2475 defines a set of DSCP values and PHBs called Class Selector (CS) PHBs which provide backward compatibility with IPP.  So if a router sets the CS DSCP value, and another devices looks only at the IPP field, it will make sense from an IPP perspective.


CS DSCP value PHBs suggest that the larger values should be given better queueing preference than packets with lower CS DSCPs.



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