In Layer 3 MPLS VPN implementations, if some of the VPNv4 routes on one PE router do not appear on another PE router, what could be the problem?

Question

Options

• ARD mismatch between the PE routers
• BRT export and import configuration errors
• CVRF name mismatch between the PE routers
• DRD export and import configuration errors

• A
  13% (6)
• B
  76% (35)
• C
  7% (3)
• D
  4% (2)

Explanation

Route Targets (RTs) control which VPNv4 routes are imported and exported between PE routers - when RT export values on the sending PE don't match the RT import values on the receiving PE, the routes simply won't be accepted into the target VRF, causing them to be missing. This is the most common real-world cause of partial route visibility in L3 MPLS VPN deployments.

Why the distractors are wrong:

• A (RD mismatch): Route Distinguishers make VPNv4 prefixes globally unique in BGP but don't control distribution - mismatched RDs would still allow routes to propagate, just with different prefixes. RDs don't have an "import" policy.
• C (VRF name mismatch): VRF names are locally significant to each PE only; there is no requirement for VRF names to match across PE routers.
• D (RD export/import): RDs are not configured with export/import policies - that concept belongs to RTs. This option confuses RD terminology with RT functionality.

Memory tip: Think RT = Route Traffic-cop - RTs decide who gets what routes. If routes are missing, the traffic-cop (RT) is blocking them. RDs are just name tags, not gatekeepers.

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