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352-001 · Question #679
352-001 Question #679: Real Exam Question with Answer & Explanation
The correct answer is A: mapDSCP bits into theExp field. In an MPLS network, QoS is enforced using the EXP field (also called the Traffic Class/TC field) in the MPLS label header, which is 3 bits wide (supporting up to 8 classes - sufficient for the required 6 classes). Why A (map DSCP into EXP) is correct: DSCP (6 bits, 64 code points
Question
You are tasked to design a QoS policy for a service provider so they can include it in the design of their MPLS core network. If the design must support an MPLS network with six classes, and CEs will be managed by the service provider, which QoS policy should be recommended?
Options
- AmapDSCP bits into theExp field
- Bmap IP precedence bits into the DSCP field
- Cmap flow-label bits into the Exp field
- Dmap IP CoS bits into the IP Precedence field
- Emap IP ToS bits into the Exp field
Explanation
In an MPLS network, QoS is enforced using the EXP field (also called the Traffic Class/TC field) in the MPLS label header, which is 3 bits wide (supporting up to 8 classes - sufficient for the required 6 classes).
Why A (map DSCP into EXP) is correct:
- DSCP (6 bits, 64 code points) is the modern, standards-based QoS marking for IP traffic per the DiffServ architecture (RFC 2474/2475).
- At the ingress PE (Provider Edge), DSCP values from incoming IP packets are mapped to MPLS EXP/TC bits. Since the CEs are SP-managed, the SP controls the DSCP markings entering the network, ensuring consistent and trusted QoS classification.
- This is the industry-standard MPLS DiffServ (RFC 3270) approach.
Why the other options are wrong:
- B: Maps IP Precedence → DSCP; this is an IP-layer reclassification that doesn't involve MPLS EXP at all.
- C: Flow label is an IPv6-specific field; irrelevant to a standard MPLS core.
- D: Maps 802.1p CoS → IP Precedence; this is L2-to-L3 mapping only, doesn't configure MPLS QoS.
- E: 'IP ToS bits' is the legacy/ambiguous predecessor to DSCP. Option A (DSCP) is the correct modern, precise answer and preferred over generic ToS.
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