350-801 Question #337: Real Exam Question with Answer & Explanation

The correct answer is B: class selector. The Class Selector (CS) Per-Hop Behavior (PHB) in DiffServ is designed to maintain backward compatibility with legacy IP Precedence schemes.

Infrastructure and Quality of Service

Question

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

Options

Aexpedited forwarding
Bclass selector
Cassured forwarding
Ddefault

Explanation

The Class Selector (CS) Per-Hop Behavior (PHB) in DiffServ is designed to maintain backward compatibility with legacy IP Precedence schemes.

Common mistakes.

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

Concept tested. DiffServ PHBs and IP Precedence compatibility

Reference. https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/WAN_and_MAN/QoS_SRND/QoS-SRND-Book/QoSDiffServ.html

Topics

#DiffServ#QoS#Class Selector#IP Precedence

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