Class Selector (CS) is a standardized DSCP marking system that preserves compatibility with the older three-bit IP Precedence (IPP) system when networks interconnect. section 10.3.3
CS uses the three most significant bits of the six-bit DSCP field—the same bits used by IPP—and sets the three least significant bits to zero, producing eight possible markings. section 10.3.3 section 10.7
Translating IP Precedence 7
IPP value 7, binary 111, corresponds to CS7. In the complete DSCP field, 111000 represents decimal value 56. section 10.3.3
Figure 10.13 The CS marking system. The most significant three bits are used to make eight possible values, and the three least significant bits are set to 0.
Use the bit-layout figure to compare the three-bit CS value with its six-bit DSCP representation. section 10.3.3
Exam-relevant facts:
- CS provides eight values because only the first three DSCP bits carry the marking. section 10.3.3
- CS is useful for organizations whose QoS policies were designed around legacy IP Precedence values. section 10.3.3
