While implementing Layer 3 MPLS VPN, which feature should an engineer use at the PEs to transform the customer IPv4 prefixes into a unique 96-bit prefix?

Question

Options

• ART
• BVC ID
• CRD
• DPW ID

• A
  3% (1)
• B
  6% (2)
• C
  81% (26)
• D
  9% (3)

Explanation

Route Distinguisher (RD) is the correct answer because it is prepended to a customer's 32-bit IPv4 prefix to create a globally unique 96-bit VPN-IPv4 (VPNv4) address (64-bit RD + 32-bit IPv4 = 96 bits). This allows PE routers to differentiate overlapping customer address spaces across the MPLS backbone using MP-BGP.

Why the distractors are wrong:

• A (RT - Route Target): RT is a BGP extended community attached to routes to control import/export policy between VRFs - it determines which routes get distributed to which customers, but it does not transform the prefix itself.
• B (VC ID - Virtual Circuit ID): VC ID is a Layer 2 concept used to identify virtual circuits in L2VPN technologies; it has no role in L3VPN prefix manipulation.
• D (PW ID - Pseudowire ID): PW ID identifies pseudowires between PE routers in Layer 2 services (e.g., VPLS); it is entirely outside the L3 MPLS VPN framework.

Memory tip: Think of RD = "Route Distinguisher" → makes prefixes distinct (unique globally), while RT = "Route Target" → controls traffic flow (import/export). RD transforms the address; RT controls where it goes.

No community discussion yet for this question.