The Differentiated Services Code Point (DSCP) value AF21 is part of the Assured Forwarding (AF) Per-Hop Behavior (PHB) group. The naming convention AFxy is encoded into a 6-bit binary value using the format xxxyy0. For AF21, the class 'x' is 2, and the drop precedence 'y' is 1.

The class value 2 is represented in 3 bits as 010.

The drop precedence value 1 (Low) is represented in 2 bits as 01.

The final bit for AF PHBs is always 0.

Combining these parts (010 01 0) gives the 6-bit binary representation 010010.

A. 010011: This binary value represents AF23 (Class 2, Drop Precedence 3), but the last bit is incorrectly set to 1 for an AF PHB.

B. 100010: This binary value represents AF41 (Class 4, Drop Precedence 1).

C. 010001: This binary value does not map to a standard AF PHB codepoint due to the final bit being 1.

D. 001100: This binary value represents AF12 (Class 1, Drop Precedence 2).

1. IETF RFC 2597, "Assured Forwarding PHB Group": Section 3, "Assured Forwarding PHB Group," defines the recommended DSCP values for AF classes. It specifies the xxxyy0 format where 'xxx' represents the class and 'yy' represents the drop precedence. The table in this section explicitly maps Class 2 to 010 and Low Drop Precedence (1) to 01, resulting in the codepoint 010010.

2. Nokia 7750 SR OS Quality of Service Guide, Release 21.10.R1: In the "QoS Overview" chapter, the section "DSCP Name-to-Value Mapping" lists the standard DSCP names and their corresponding values. It maps af21 to a decimal value of 18, which is 010010 in 6-bit binary representation. (Document ID: 3HE 17279 AAAC TQZZA, Page 55).