Which DiffServe PHB preserves backward compatibility with any IP precedence scheme?

Expedited Forwarding (EF) has a specific DSCP value (46) that does not directly map to an IP Precedence value for backward compatibility.

Class Selector (CS) PHBs are specifically defined within the DiffServ architecture to be backward compatible with the older IP Precedence (IPP) scheme. The first three bits of the DSCP field in CS values are structured such that they directly correspond to the IP Precedence value, allowing legacy devices to interpret the priority correctly.

Assured Forwarding (AF) PHBs use the full 6-bit DSCP field to define classes and drop precedences, which does not directly preserve the IP Precedence bits in a backward-compatible manner.

The Default PHB (DSCP 0) maps to IP Precedence 0, but the Class Selector PHB group provides comprehensive backward compatibility across all IP Precedence values.