350-501 Services Practice Question
An ISP operates an MPLS Layer 3 VPN backbone. A customer, Corporation X, has four sites (A, B, C, D) each connected to a different PE router (PE-A, PE-B, PE-C, PE-D). They use OSPF as the CE-PE routing protocol. Sites A, B, and C can exchange routes without issue. However, Site D suddenly loses connectivity to Site B, though it can still reach Sites A and C. Show commands on PE-D reveal that the VRF for Corporation X imports RT:100:200, and the route for Site B (prefix 10.10.20.0/24) is present in the BGP VPNv4 table with RT:100:200 and next-hop 10.0.2.2. The OSPF neighbor between PE-D and the CE at Site D is up, and no route filters are configured. ‘ping 10.10.20.0’ from PE-D fails. What is the most likely cause of the issue?
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 (10.0.2.2) for Site B's route is not reachable in PE-D's global routing table.
The route is in the VPNv4 table with the correct RT, so import is working. The next-hop is 10.0.2.2. For the VPNv4 route to be installed into the VRF, the next-hop must be reachable in the global routing table of PE-D. If it is not reachable, the route will not be installed, explaining the loss of connectivity to Site B. This can be verified with 'show ip route 10.0.2.2' on PE-D. Option A is incorrect because OSPF route type does not affect BGP redistribution. Option B is incorrect because the route is present in the VPNv4 table with the correct RT, so import is fine. Option C is incorrect because label allocation is working (the prefix is present with labels).
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
OSPF route redistribution from PE-B into BGP is set to external type 2, while PE-D expects type 1.
Why it's wrong here
Incorrect: OSPF route type does not affect BGP import or next-hop reachability; all OSPF routes are treated similarly.
- ✗
The route target for Site B is misconfigured on PE-D, not matching the export RT from PE-B.
Why it's wrong here
Incorrect: The VPNv4 table shows the route with the correct RT, so import is not the issue.
- ✗
PE-D is not assigning an MPLS label for the route to Site B, causing packets to be dropped.
Why it's wrong here
Incorrect: The route in the VPNv4 table shows mpls labels in/out, so label allocation is intact.
- ✓
The BGP next-hop (10.0.2.2) for Site B's route is not reachable in PE-D's global routing table.
Why this is correct
Correct: If the next-hop is unreachable, the VPNv4 route cannot be installed in the VRF, causing loss of connectivity to that specific site.
Visual reference
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.