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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 cef vrf CUSTOMER 10.2.2.0 command on PE1 shows the next-hop as 192.168.1.2 and the output interface as GigabitEthernet0/0. The show mpls forwarding-table 192.168.1.2 detail command on PE1 shows a label but the outgoing interface is 'aggregate'. 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 PE2 loopback address is accidentally configured on PE1.
The label for the BGP next-hop is pointing to 'aggregate', which means the router is the egress LSR for that prefix. This occurs when the PE2 loopback is also configured on PE1, causing the router to think it is the destination. The traffic is then dropped or looped because the router tries to process the packet locally instead of forwarding it.
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 PE2 loopback address is accidentally configured on PE1.
Why this is correct
Correct: If PE1 has the same loopback IP, it will treat itself as the egress for that prefix, causing 'aggregate' in the LFIB.
- ✗
LDP is not enabled on the core-facing interfaces.
Why it's wrong here
Incorrect because a label is present, so LDP is working.
- ✗
The VRF route-target import is misconfigured.
Why it's wrong here
Incorrect because the route is installed in the VRF.
- ✗
The MP-BGP session is using the wrong update-source.
Why it's wrong here
Incorrect because the route is received and installed.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
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.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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