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Which of the following decimal values corresponds to the DSCP value EF? (Select the best answer.)

Which of the following decimal values corresponds to the DSCP value EF? (Select the best answer.)

A. 0
B. 14
C. 28

D. 46

Explanation:

The decimal value 46 corresponds to the Differentiated Services Code Point (DSCP) value EF. DSCP values are 6bit header values that identify the Quality of Service (QoS) traffic class that is assigned to the packet? converting the decimal value 46 to binary yields the 6bit binary value 101110. The Expedited Forwarding (EF) perhop behavior (PHB), which is defined in Request for Comments (RFC) 2598, indicates a high priority packet that should be given queuing priority over other packets but should not be allowed to completely monopolize the interface. Voice over IP (VoIP) traffic is often assigned a DSCP value of EF.
The decimal value 0 corresponds to the default DSCP value CS0. DSCP values beginning with CS are called Class Selector (CS) PHBs, which are defined in RFC 2475. CS values are backward compatible with 3bit IP precedence values? the first three bits of the DSCP value correspond to the IP precedence value, and the last three bits of the DSCP value are set to 0. Packets with higher CS values are given queuing priority over packets with lower CS values. The following table displays the CS values with their binary values, decimal values, and IP precedence category names:

The decimal value 14 corresponds to the DSCP value AF13, and the decimal value 28 corresponds to the DSCP value AF32. DSCP values beginning with AF are called Assured Forwarding (AF) PHBs, which are defined in RFC 2597. AF separates packets into four queue classes and three drop probabilities. The AF values are specified in the format AFxy, where x is the queue class and y is the drop probability. The following table displays the AF values with their queue classes and drop rates:

The first three DSCP bits correspond to the queue class, the fourth and fifth DSCP bits correspond to the drop probability, and the sixth bit is always set to 0. To quickly convert AF values to decimal values, you should use the formula 8x + 2y. For example, AF13 converts to a decimal value of 14, because (8 x 1) + (2 x 3) = 8 + 6 = 14. The DSCP value AF32 converts to a decimal value of 28, because (8 x 3) + (2 x 2) = 24 + 4 = 28.
Packets with higher AF values are not necessarily given preference over packets with lower AF values. Packets with a higher queue class value are given queuing priority over packets with a lower queue class, but packets with a higher drop rate value are dropped more often than packets with a lower drop rate value.

Reference:
Cisco: Implementing Quality of Service Policies with DSCP: Expedited Forwarding


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