In an MPLS-VPN environment, what is the effect of configuring an identical set of route targets for a particular VRF, but then configuring nonidentical route distinguisher across multiple PE devices?

Question

Options

• AThe routes will be correctly handled by the control plane, but there will be instances where routes
• BThe routes will propagate to the remote PE, but the PE will never install them in its forwarding table.
• CThe routes will be rejected by the remote PE because they have a different RD than its routes.
• DThe routes will not even be sent to any remote PE with a different RD.

• A
  55% (23)
• B
  5% (2)
• C
  14% (6)
• D
  26% (11)

Explanation

Route Distinguishers (RDs) and Route Targets (RTs) serve distinct roles in MPLS VPN. The RT governs import/export policy - which routes enter or leave a VRF. The RD makes routes globally unique in the BGP table by constructing a 96-bit VPNv4 prefix (RD + IPv4 prefix). When PEs share identical RTs but use nonidentical RDs, the control plane still functions correctly: routes are exported and imported based solely on RT matching, regardless of the RD value. However, because each PE's VPNv4 prefix for the same customer route is different (different RD component), BGP treats them as distinct routes. This can cause suboptimal behavior in certain scenarios - for example, a Route Reflector may advertise only one best path, or multipath load balancing behavior may differ. Crucially, routes are neither rejected nor silently dropped, ruling out choices C and D. Choice B is also wrong because the PE will install routes with matching RTs into its forwarding table regardless of the RD. Choice A correctly captures that the control plane handles these routes but edge-case issues can arise.

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