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300-410 Practice Question: An engineer is troubleshooting an MPLS L3VPN…
An engineer is troubleshooting an MPLS L3VPN where CE1 (10.1.1.0/24) cannot reach CE2 (10.2.2.0/24). The PE routers have MP-BGP peering and the VRF is configured with route-target import 100:100. 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, but the show ip route vrf CUSTOMER command does not have this route. The show ip bgp vpnv4 all 10.2.2.0/24 command on PE1 shows the route is received but not best. 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 BGP next-hop (PE2 loopback) is not reachable in the global routing table.
The route is received but not marked as best, so it is not installed in the routing table. Common reasons include the route being suppressed due to a higher AD from another source or the next-hop being unreachable. In this scenario, the most likely cause is that the BGP next-hop is not reachable in the global routing table.
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 import on PE1 is missing.
Why it's wrong here
Incorrect because the route is received, so import is working.
- ✓
The BGP next-hop (PE2 loopback) is not reachable in the global routing table.
Why this is correct
Correct: BGP requires the next-hop to be reachable for the route to be considered best and installed.
- ✗
The VRF on PE1 has a different route-target export.
Why it's wrong here
Incorrect because the route is received, indicating export is correct.
- ✗
The MP-BGP session is using an incorrect address family.
Why it's wrong here
Incorrect because the route is received in the VPNv4 address family.
Go deeper
Related to this question
Learn chapter
OSPF Route Summarization and Filtering
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
Senior Network & Security Engineer · founder of Courseiva
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