300-410 Layer 3 Technologies Practice Question
A network engineer is deploying MPLS Layer 3 VPNs. The engineer must ensure that the PE routers can forward VPN traffic correctly. The following configuration is applied on a PE router:
ip vrf CUSTOMER
rd 65000:1 route-target export 65000:1 route-target import 65000:1 !
interface GigabitEthernet0/1 ip vrf forwarding CUSTOMER ip address 192.168.1.1 255.255.255.0
After configuration, the engineer notices that the CE router cannot reach remote sites. The MPLS core is operational, and MP-BGP is configured. What is the most likely missing configuration?
⚠ Common exam trap
The trap here is focusing on the VRF and interface configuration while overlooking the need for a BGP address-family per VRF to exchange VPN routes.
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
✓
The BGP process must be configured with address-family ipv4 vrf CUSTOMER and the necessary redistribution or network statements.
In MPLS L3 VPN, the PE router must have a BGP address-family ipv4 vrf for each VRF to exchange VPN routes with other PE routers. The VRF definition and interface assignment are necessary but not sufficient. Without the address-family configuration and route redistribution or network statements, the PE will not advertise the customer routes, and remote sites will be unreachable.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The route-target must be configured under the interface GigabitEthernet0/1.
Why it's wrong here
Route targets are configured under the VRF definition, not on the interface. The interface only needs ip vrf forwarding to associate the interface with the VRF. The missing configuration is the BGP address-family for the VRF.
- ✗
The PE router must have a route distinguisher configured under the BGP process.
Why it's wrong here
The route distinguisher is configured under the VRF definition, not under the BGP process. It is already present as rd 65000:1. The missing piece is not related to the RD; it is the lack of a loopback interface for BGP peering.
- ✗
The PE router must have a loopback interface configured and used for BGP peering.
Why it's wrong here
While a loopback interface is recommended for BGP peering stability, its absence alone would not prevent VPN traffic forwarding if BGP peering is established via physical interfaces. The more critical missing configuration is the address-family ipv4 vrf under BGP.
- ✓
The BGP process must be configured with address-family ipv4 vrf CUSTOMER and the necessary redistribution or network statements.
Why this is correct
For MPLS L3 VPN, the PE router must have a BGP address-family for each VRF to exchange routes with other PEs. Without address-family ipv4 vrf CUSTOMER, the PE will not advertise or receive VPN routes for that customer, preventing connectivity. The VRF configuration alone is insufficient.
Go deeper
Related to this question
Learn chapter
BGP Route Selection and Policy
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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