300-420 · Question #110
A network engineer is redesigning a company's QoS solution. The company is currently using IP Precedence, but the engineer plans to move to DiffServ. It is important that the new solution provide…
The correct answer is C. class selector code points. Class Selector (CS) code points are specifically designed to provide backward compatibility between IP Precedence and DiffServ. IP Precedence uses the top 3 bits of the ToS byte (values 0–7). DiffServ Class Selector PHBs (CS0 through CS7) are defined so that the first 3 bits of…
Question
A network engineer is redesigning a company's QoS solution. The company is currently using IP Precedence, but the engineer plans to move to DiffServ. It is important that the new solution provide backward compatibility with the current solution. Which technology should the design include?
Options
- Aexpedited forwarding
- Bassured forwarding
- Cclass selector code points
- Ddefault per hop behavior
How the community answered
(25 responses)- A4% (1)
- B8% (2)
- C88% (22)
Explanation
Class Selector (CS) code points are specifically designed to provide backward compatibility between IP Precedence and DiffServ. IP Precedence uses the top 3 bits of the ToS byte (values 0–7). DiffServ Class Selector PHBs (CS0 through CS7) are defined so that the first 3 bits of the 6-bit DSCP field match the corresponding IP Precedence value, with the remaining 3 bits set to zero. This means a device using IP Precedence and a device using DiffServ CS code points will interpret the same marked bits consistently, preserving interoperability during a phased migration. Option A (Expedited Forwarding / EF) and Option B (Assured Forwarding / AF) are DiffServ PHBs without IP Precedence compatibility. Option D (default PHB) corresponds to best-effort forwarding (DSCP 0) and provides no classification.
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