Last Updated on August 7, 2021 by Admin 3
Which of the following statements is true about DiffServ class AF41? (Select the best answer.)
- AF41 has a low priority and a low drop probability.
- AF41 has a low priority and a high drop probability.
- AF41 has a high priority and a low drop probability.
- AF41 has a high priority and a high drop probability.
DiffServ class AF41 has a high priority and a low drop probability. AF41 is a Differentiated Services Code Point (DSCP) value, which is a 6bit header value that identifies the Quality of Service (QoS) traffic class that is assigned to the packet. DSCP values beginning with AF are called Assured Forwarding (AF) perhop behaviors (PHBs), which are defined in Request for Comments (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:
AF11 has a low priority and a low drop probability. AF13 has a low priority and a high drop probability. AF43 has a high priority and a high drop probability.
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, AF41 converts to a decimal value of 34, because (8 x 4) + (2 x 1) = 32 + 2 = 34.
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.