A packet is to be encapsulated inside an MPLS tunnel which consists of two MPLS headers at the first network egress. Which of the MPLS EXP fields will be marked?

Question

Options

• AThe EXP field in the bottom MPLS header will be marked.
• BThe EXP field in both the top and bottom MPLS headers will be marked.
• CFor Layer 2 services, the top MPLS header will be marked; for Layer 3 services, the bottom
• DThe EXP field in the top MPLS header will be marked.
• ENeither the top nor the bottom EXP field will be marked since EXP remarking can only be done on

• B
  93% (27)
• C
  3% (1)
• D
  3% (1)

Explanation

Option B is correct because when a packet enters an MPLS tunnel with a two-label stack, QoS marking must be applied to both EXP fields simultaneously at ingress - the top (tunnel) label's EXP ensures intermediate P-routers apply correct QoS treatment in transit, while the bottom (service) label's EXP preserves the original QoS marking so the egress PE can honor it after label popping.

Why the distractors fail:

• A (bottom only): Intermediate P-routers only inspect the top label, so marking only the bottom label leaves them blind to QoS intent - traffic would be forwarded with default priority through the core.
• C (Layer 2 vs. Layer 3 split): The service type doesn't dictate which EXP fields are marked; both are always marked regardless of L2 or L3 service.
• D (top only): The egress PE needs the bottom label's EXP to restore the original QoS marking after the tunnel label is removed - marking only the top loses that information.
• E (neither): Incorrect - EXP remarking is a core MPLS QoS mechanism and is absolutely performed at ingress PE nodes.

Memory tip: Think "two labels, two marks" - a two-label stack is like a sealed envelope inside a mailing envelope; you stamp both for the postal service (P-routers) to handle it correctly and for the recipient (egress PE) to know its priority after opening.

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