352-001 Question #92: Real Exam Question with Answer & Explanation

Question

Options

• AMPLS QoS is a scalable and simple solution because it only requires application at the Edge-LSR.
• BMPLS can force packets into specific paths and bandwidths based on Edge-LSR mapping of
• CDiffServ information must be made known to the LSR in the MPLS header using the EXP field.
• DOn egress from the Edge-LSR, the EXP field is used for PHB QoS towards the CE.

Explanation

Two accurate statements about QoS in MPLS VPN (RFC 2547bis): (B) MPLS Traffic Engineering (TE) allows the Edge-LSR to map packets onto specific Label Switched Paths (LSPs) with reserved bandwidths, effectively forcing traffic into predetermined paths - this is a core capability of MPLS-TE at the ingress PE. (C) Core LSRs in an MPLS network have no visibility into the IP header (it is encapsulated beneath one or more labels), so DiffServ PHB information must be encoded in the EXP (now called TC - Traffic Class) field of the MPLS label header for core LSRs to apply QoS treatment. Choice A is incorrect because QoS must be applied at core LSRs as well (using the EXP/TC field), not only at Edge-LSRs - applying classification only at the edge while ignoring core LSR behavior is insufficient. Choice D is incorrect because on egress from the Edge-LSR (PE), MPLS labels are popped and the EXP field ceases to exist; QoS treatment toward the CE is based on the IP DSCP field, not the EXP field.

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