6.2.a Implement and troubleshoot end-to-end QoS

6.2.a [i] CoS and DSCP mapping

class selector is identified by the  3 msb’s (most significant bits) of the dscp byte in the ip header

000 through 111 =  0 (default) on up to 7    000 = 0      111 = 4 2 1 = 7

the class selector’s PHB (per hop behavior) is simply that a higher value CS will be given queueing preference

assured forwarding PHB defines four classes for queueing, and three drop priorities in each AF. unlike the CS value (higher preferred), the higher numbered drop priority is more likely dropped, ie:

whereas CS4 is preferred over CS3, within CS4, AF43 is more likely dropped than AF41

the first number in the AF PHB identifies the queue, the second number is the drop likelihood

the formula AFxy can be used to identify the decimal value of the AF PHB, to wit:

8x + 2y converts AF31 to 8(3) + 2(1) = 24 + 2 = 26

8x + 2y converts AF42 to 8(4) + 2(2) = 32 + 4 = 36

see chart below:


note rfc 2598 defines EF (expedited forwarding, decimal value 46) as performing two PHB actions:

Queue EF packets for quick scheduling and low latency

Police EF packets so they do not consume all available bandwidth or starve other queues