300-410 Infrastructure Services Practice Question
A network engineer is configuring a Cisco IOS XE router to support MPLS Layer 3 VPNs. The engineer has enabled MPLS IP on the core interfaces and configured OSPF as the IGP. Which additional configuration is required on the PE routers to exchange VPNv4 routes with other PE routers?
⚠ Common exam trap
The trap here is assuming that MPLS LDP or IGP configurations are sufficient for VPN route exchange, when MP-BGP is the key component.
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 with the `address-family vpnv4` configuration and activate neighbors.
MPLS Layer 3 VPNs rely on MP-BGP to distribute VPNv4 routes among PE routers. Enabling the VPNv4 address family and activating neighbors under that address family is mandatory for exchanging customer VPN routes with the associated route targets and labels. Without this BGP configuration, PE routers cannot learn each other's VPN routes, and the VPN will not operate.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Enable BGP with the `address-family vpnv4` configuration and activate neighbors.
Why this is correct
MPLS Layer 3 VPNs require MP-BGP to exchange VPNv4 routes between PE routers. The `address-family vpnv4` configuration enables the VPNv4 address family, and neighbors must be activated within that address family. This allows the PE routers to advertise customer routes with route targets and VPN labels. Without this, VPNv4 routes cannot be exchanged, and the VPN will not function.
- ✗
Enable `mpls ip` on all PE-CE interfaces.
Why it's wrong here
MPLS IP is typically enabled on core-facing interfaces to establish LDP sessions and distribute labels for core routes. It is not required on PE-CE interfaces because the PE-CE link is usually an IP link without MPLS. Enabling MPLS on PE-CE interfaces is unnecessary and does not facilitate VPNv4 route exchange between PE routers.
- ✗
Configure `mpls ldp router-id Loopback0` to ensure LDP uses the loopback interface.
Why it's wrong here
While configuring a stable router ID for LDP is good practice, it is not the additional configuration required to exchange VPNv4 routes. LDP is used for label distribution for the IGP routes, but VPNv4 route exchange relies on MP-BGP. Setting the LDP router ID does not enable VPNv4 route exchange and is not sufficient for MPLS L3 VPN operation.
- ✗
Configure a route reflector for the IGP to distribute routes between PE routers.
Why it's wrong here
A route reflector is used within BGP to reduce the number of IBGP sessions, but it is not a requirement for basic VPNv4 route exchange. The IGP (OSPF) is used for internal reachability, not for VPNv4 routes. While a route reflector might be used in larger deployments, it is not the additional configuration needed to enable VPNv4 exchange; MP-BGP configuration is essential.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
MPLS L3VPN Basics
Key term
MP-BGP for VPN
MP-BGP for VPN is an extension of the Border Gateway Protocol that carries multiple types of network layer information to enable scalable, isolated virtual private networks over a shared provider infrastructure.
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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