300-410 Infrastructure Services Practice Question
A network administrator is deploying MPLS Layer 3 VPNs across a service provider backbone. The provider uses OSPF as the IGP and MP-BGP for VPNv4 route distribution. The administrator notices that VPNv4 routes are not being advertised between PE routers. Which configuration step is most likely missing on the PE routers?
⚠ Common exam trap
The trap here is assuming that MPLS LDP or OSPF configuration alone is sufficient for VPNv4 route exchange, overlooking the need for BGP address family activation.
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
✓
Activating the VPNv4 address family under the BGP routing process and configuring the PE routers as neighbors
MPLS L3VPN relies on MP-BGP to distribute VPNv4 routes between PE routers. The VPNv4 address family must be activated under the BGP process, and PE routers must be configured as neighbors within that address family. Without this, VPNv4 NLRIs are not exchanged, and customer routes cannot be propagated across the provider backbone.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configuring MPLS LDP on all interfaces between PE and P routers
Why it's wrong here
MPLS LDP is necessary for label distribution and forwarding, but it does not enable VPNv4 route advertisement. Without LDP, MPLS forwarding would fail, but the symptom described is about route advertisement, not label forwarding. LDP is a separate requirement from BGP VPNv4 configuration.
- ✗
Enabling OSPF on the PE-CE links with the correct area ID
Why it's wrong here
OSPF on PE-CE links is used for customer route exchange, not for VPNv4 route distribution between PEs. While OSPF is important for the customer VRF, it does not enable MP-BGP VPNv4 peering. The missing step is likely related to BGP configuration, not OSPF.
- ✗
Configuring route reflectors or full mesh iBGP peering between PE routers
Why it's wrong here
While iBGP full mesh or route reflectors are necessary for scaling, the question implies a more fundamental missing step. If VPNv4 routes are not advertised at all, the issue is likely the address family activation. Route reflectors are a scaling mechanism, not the initial enabler of VPNv4 exchange.
- ✓
Activating the VPNv4 address family under the BGP routing process and configuring the PE routers as neighbors
Why this is correct
For VPNv4 routes to be exchanged, the BGP process must have the VPNv4 address family activated and the PE routers must be configured as BGP neighbors within that address family. Without this, MP-BGP will not carry VPNv4 NLRIs. This is the fundamental step for MPLS L3VPN route distribution.
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
Device Management and Network Monitoring (SNMP, Syslog, NetFlow)
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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