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300-410 Practice Question: Is troubleshooting an MPLS L3VPN where CE1…
A network engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers are using eBGP with the CEs. On PE1, the show ip bgp vpnv4 vrf CUSTOMER command shows the route for 10.2.2.0/24 with a next-hop of 192.168.1.2, and the show ip route vrf CUSTOMER command shows the route. However, traffic from CE1 to CE2 fails. The show ip bgp vpnv4 vrf CUSTOMER 10.2.2.0/24 command on PE1 shows the route is received and best, but the show ip bgp vpnv4 vrf CUSTOMER 10.2.2.0/24 neighbors 192.168.1.2 advertised-routes command on PE2 shows the route is not advertised. What is the most likely cause?
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 VRF on PE2 does not have the CE2 route in its routing table.
The route is not being advertised from PE2 to PE1. This could be due to a missing network statement or redistribute command under the BGP VRF address family on PE2, or the route is not in the VRF routing table on PE2. The engineer should check PE2's VRF routing table for the CE2 route.
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 VRF on PE2 does not have the CE2 route in its routing table.
Why this is correct
Correct: If the route is not in the VRF routing table on PE2, it cannot be advertised via BGP.
- ✗
The MP-BGP session between PE1 and PE2 is down.
Why it's wrong here
Incorrect because the route is received on PE1, so the session is up.
- ✗
The route-target export on PE2 is misconfigured.
Why it's wrong here
Incorrect because the route is received, so export is working.
- ✗
The VRF on PE1 has the wrong route-target import.
Why it's wrong here
Incorrect because the route is received on PE1.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
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
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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Written by Johnson Ajibi, MSc IT Security
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