Which Protocol Advertises PE Routes to a CE Router in MPLS L3VPN?
Short answer
B BGP IPv4 advertises the RD-stripped prefix from the PE to the CE inside the VRF with no redistribution or static routes.
Question
Key Takeaways
The question tests whether you can separate the PE-to-PE VPNv4 transport (MP-BGP) from the PE-to-CE advertisement (BGP IPv4 in the VRF); the trap is picking MP-BGP simply because the scenario is about MPLS VPN and RD/RT tags.
In an MPLS Layer 3 VPN, VPNv4 prefixes carried by MP-BGP between PE routers are stripped of their RD before being handed to a VRF-unaware CE, and that PE-to-CE advertisement is made with BGP IPv4, with no redistribution or static routes required. This page confirms why option B, not MP-BGP VPNv4, is the correct answer.
Common mistake: Choosing MP-BGP (D) because VPNv4 with RD/RT extended communities seems to be the MPLS L3VPN protocol; however the CE is VRF-unaware and never joins the VPNv4 address family, so once the RD is removed the PE uses the plain BGP IPv4 address family toward the CE.
Expert Analysis
Why the Answer Is Correct
The PE router advertises routes into the VRF toward the CE, and because the RD (with the RT) has already been stripped, what remains is an ordinary IPv4 prefix. That advertisement is performed with the BGP IPv4 address family under the VRF (address-family ipv4 vrf <name>, neighbor x.x.x.x activate), which is exactly what option B describes. Because the same protocol instance that receives the route from one CE can deliver it to the far-end CE, no redistribution and no static routes are needed anywhere in the path, satisfying the condition stated in the question. MP-BGP/VPNv4 remains only the PE-to-PE transport; it is not the protocol that hands the prefix to the CE.
Why the Other Options Are Wrong
OSPF (A) and IS-IS (C) are IGPs that have no concept of VRFs, route distinguishers or route targets, so using them toward the CE would force redistribution of VPN routes into the IGP on the PE, which the question explicitly forbids. MP-BGP (D) is the strongest distractor: it is the protocol that carries the VPNv4 prefixes with their RD and RT extended communities, but that session runs between PE routers and is configured in the vpnv4 address family. The CE is not VRF-aware and cannot participate in that address family, so after the RD is removed the PE uses plain BGP IPv4 for the PE-to-CE session.
Community Comment Notes
The 80-to-20 vote split for B over D matches the exam key and the majority reading of the question. Sammy3637 selected D, arguing that "BGP IPv4 doesn't support the VPNv4 address family", but that objection belongs to the PE-to-PE session; the scenario asks specifically about the PE-to-CE advertisement, where the RD has already been removed and a plain IPv4 prefix is all that is left. d740f62 answered B and linked a Cisco community MPLS VPN configuration walkthrough showing the PE-CE BGP setup, and Pietjeplukgeluk simply stated "B seems ok". No commenter showed any evidence that the CE ever joins the VPNv4 address family.
Official Reference
https://community.cisco.com/t5/networking-knowledge-base/how-to-configure-mpls-vpn/ta-p/3108825
This link opens the official vendor documentation in a new tab.
Exam Strategy
When a stem says "without redistribution or static," look for the protocol that can carry the route end-to-end in the same process, which here is BGP, and then check the direction of the session before naming it. Also remember that the P router never inspects the RD at all, so the phrasing about removing the RD should point you to the egress PE and the CE-facing session.
Frequently Asked Questions
Why isn't MP-BGP the protocol that advertises routes to the CE?
Does the P router remove the RD before forwarding the route?
Community Votes
80% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 3 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 3
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Sammy3637 👍 1 Answered DBGP IPv4 doesn't support the VPNv4 address family, which is needed for advertising routes that include customer-specific information (like the RD and RT tags). MP-BGP (Option D) is the protocol that supports the additional features required to advertise VPN routes correctly, even after the RD is removed from the P router.
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d740f62 👍 1 Answered Bhttps://community.cisco.com/t5/networking-knowledge-base/how-to-configure-mpls-vpn/ta-p/3108825#:~:text=The%20PE%20routers%20exchange%20these,packets%20across%20the%20P%20network.
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Pietjeplukgeluk 👍 2 Answered BB seems ok
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