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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 are using OSPF 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 ip route 192.168.1.2 command on PE1 shows the route with a next-hop of 10.0.0.2 and output interface GigabitEthernet0/0. The show mpls forwarding-table 192.168.1.2 detail command on PE1 shows a label with outgoing interface GigabitEthernet0/0. 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 CE1 router does not have a default route or specific route to 10.2.2.0/24.
All forwarding components appear correct: the route is in the VRF, CEF has a valid next-hop and interface, and MPLS has a label. The issue is likely on the CE side, such as a missing route on CE1 or a firewall blocking traffic. The engineer should check CE1's 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 VRF route-target import on PE1 is misconfigured.
Why it's wrong here
Incorrect because the route is installed in the VRF.
- ✓
The CE1 router does not have a default route or specific route to 10.2.2.0/24.
Why this is correct
Correct: If CE1 does not have a route to the remote prefix, it will drop traffic or send it to a default gateway that may not exist.
- ✗
The OSPF process on PE1 is not redistributing BGP routes into OSPF.
Why it's wrong here
Incorrect because the route is in the VRF routing table, and OSPF redistribution is for CE-PE, not for forwarding.
- ✗
The MP-BGP session is not using the loopback interface.
Why it's wrong here
Incorrect because the route is received and installed.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
OSPF Neighbor Adjacency and Basics
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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