300-410 Infrastructure Services Practice Question
A network engineer is troubleshooting an MPLS L3VPN where customer routes are not being propagated between PE routers. The engineer verifies that the MP-BGP session between the PEs is established and that VRFs are configured correctly. Which of the following is the most likely cause for the missing routes?
⚠ Common exam trap
The trap here is assuming that an established MP-BGP session and correct VRF configuration guarantee route propagation, overlooking the role of route targets.
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 route targets are not properly configured for import/export.
In MPLS L3VPN, route targets are extended BGP communities that determine which VRFs import and export routes. For a route to be installed in a remote VRF, the export RT attached to the route by the originating PE must match an import RT configured on the receiving PE's VRF. If they do not match, the route is discarded. This is a frequent issue when VRFs are newly configured or when RTs are changed. Verifying RT configuration on both PEs is essential.
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 targets are not properly configured for import/export.
Why this is correct
In MPLS L3VPN, route targets (RTs) control the import and export of VPNv4 routes between VRFs. If the export RT on one PE does not match the import RT on the remote PE, the routes will not be imported into the remote VRF, even though the MP-BGP session is up and the routes are exchanged. This is a common misconfiguration and directly causes missing customer routes.
- ✗
The MPLS LDP session between the PEs is down.
Why it's wrong here
LDP is used for label distribution for the transport label, but MP-BGP is used for VPNv4 route exchange. If LDP is down, MPLS forwarding may fail, but the VPNv4 routes would still be present in BGP. The symptom described is missing routes, not forwarding failure. Moreover, LDP sessions are typically between directly connected routers, not necessarily between PEs.
- ✗
The PE routers are not configured with unique route distinguishers.
Why it's wrong here
Route distinguishers (RDs) are used to make VPNv4 prefixes unique. If two PEs used the same RD for different VRFs, it could cause conflicts, but typically each VRF on a PE has a unique RD. The scenario says VRFs are correctly configured, implying RDs are set. Even with duplicate RDs, routes might still be propagated but could be misinterpreted.
- ✗
The BGP session is not configured with the correct address-family.
Why it's wrong here
The scenario states that the MP-BGP session is established and VRFs are configured. If the address-family were incorrect, the session might not exchange VPNv4 routes at all, but the engineer already verified the session is up. While address-family misconfiguration is possible, it would likely prevent any VPNv4 route exchange, not just selective routes.
Go deeper
Related to this question
Learn chapter
OSPF Route Summarization and Filtering
Key term
BGP Communities
BGP communities are tags attached to routes that allow routers to apply common policies across networks without manual per-route configuration.
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.
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JA
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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