352-001 · Question #756
352-001 Question #756: Real Exam Question with Answer & Explanation
The correct answer is C: QoS marking establishes a trust boundary that scheduling tools later depend on. C is correct: QoS marking (or re-marking) at trust boundaries assigns standardized values (e.g., DSCP, CoS) that downstream QoS mechanisms-queuing, scheduling, policing-rely on to make forwarding decisions. Without consistent, trusted markings, scheduling tools cannot correctly d
Question
Options
- AShaping is one of the ways that packets can be remarked.
- B802.1Q/p CoS bits and IP Precedence are Layer 3 marking fields.
- CQoS marking establishes a trust boundary that scheduling tools later depend on
- DMPLS EXP and DSCP are Layer 2 marking fields
- EClass-based marking occurs after packet classification
Explanation
C is correct: QoS marking (or re-marking) at trust boundaries assigns standardized values (e.g., DSCP, CoS) that downstream QoS mechanisms-queuing, scheduling, policing-rely on to make forwarding decisions. Without consistent, trusted markings, scheduling tools cannot correctly differentiate traffic classes. E is correct: class-based marking is a two-step process-packets must first be classified (matched against ACLs, NBAR, or other criteria) before a marking action can be applied. Classification always precedes marking. The other options are incorrect: A is wrong because shaping buffers/delays traffic to enforce a rate; it does not remark packets. B is wrong because 802.1Q/p CoS bits are a Layer 2 field (Ethernet frame header), not Layer 3-only IP Precedence is Layer 3. D is wrong because DSCP is a Layer 3 field (IP header), while MPLS EXP (now called TC bits) operates at Layer 2.5 in the MPLS label stack, not strictly Layer 2.
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