300-410 Layer 3 Technologies Practice Question
A network engineer is configuring MPLS Layer 3 VPN on a Cisco IOS XR router. The engineer needs to ensure that customer routes are properly propagated across the MPLS core. Which two of the following are required to establish the VPNv4 peering between PE routers? (Choose two.)
⚠ Common exam trap
Candidates often confuse the transport plane (LDP/IGP) with the control plane for VPNs (BGP VPNv4), and thinking that LDP or IGP is required for VPNv4 peering.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable BGP address-family vpnv4 unicast on the PE routers.
To establish VPNv4 peering between PE routers, you must enable the BGP VPNv4 address family and have either a full mesh of iBGP sessions or a route reflector. These two elements allow the exchange of VPNv4 routes with route distinguishers and route targets, which are essential for MPLS L3VPN. LDP and IGP are for transport but not for VPNv4 peering itself.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure OSPF as the IGP for the MPLS core.
Why it's wrong here
An IGP such as OSPF is needed for reachability of the PE router loopbacks and for LDP to establish transport LSPs, but it is not directly required for VPNv4 peering. VPNv4 peering is a BGP function. While an IGP is necessary for the core, the question focuses on VPNv4 peering, so this is not a direct requirement.
- ✓
Enable BGP address-family vpnv4 unicast on the PE routers.
Why this is correct
To exchange VPNv4 routes between PE routers, BGP must be configured with the VPNv4 address family. This allows the PE routers to carry customer routes with route distinguishers and route targets. Without enabling the VPNv4 address family, the PE routers cannot exchange VPN routing information, and MPLS L3VPN would not function.
- ✗
Enable LDP for label distribution on the core interfaces.
Why it's wrong here
LDP is used to distribute labels for the transport label (IGP labels) in the MPLS core, but it is not used for VPNv4 route exchange. While LDP is required for the underlying MPLS forwarding, it is not part of establishing VPNv4 peering. The question specifically asks for VPNv4 peering requirements, so LDP is not the correct choice.
- ✗
Enable BGP address-family ipv4 unicast for customer routes.
Why it's wrong here
The IPv4 unicast address family is used for global routing or for PE-CE routing, but it is not used for VPNv4 peering between PE routers. VPNv4 peering specifically uses the VPNv4 address family. Enabling IPv4 unicast alone would not establish the VPNv4 session or exchange VPN routes.
- ✓
Configure a route reflector or full mesh of iBGP sessions between PE routers.
Why this is correct
VPNv4 routes are exchanged via iBGP between PE routers. To scale, a route reflector is often used to avoid a full mesh. Without either a full mesh of iBGP sessions or a route reflector, PE routers cannot propagate VPNv4 routes to each other, preventing end-to-end VPN connectivity.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
MPLS Layer 3 VPN
A technology that uses Multiprotocol Label Switching to create secure, scalable virtual private networks that connect multiple sites at the network layer, where the service provider manages routing between customer sites.
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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