350-501 MPLS and Segment Routing Practice Question
An engineer is troubleshooting an MPLS VPN issue where CE1 cannot ping CE2 across the provider network. The PE routers are configured with MP-BGP and LDP. On PE1, 'show ip bgp vpnv4 vrf CUST' shows the route to CE2's loopback as valid, but 'show mpls forwarding-table' does not list a label for that prefix. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between BGP route validity (learned via MP-BGP) and VRF route installation (controlled by RT import), leading candidates to mistakenly suspect BGP session or LDP issues when the real problem is a route target mismatch.
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
✓
Route target mismatch between PE1 and PE2
The route is present in the VRF BGP table on PE1 (valid), but no MPLS label is assigned for that prefix in the forwarding table. This indicates that PE1 has learned the route via MP-BGP but cannot install it into the MPLS forwarding table because the route target (RT) import policy on PE1 does not match the RT export policy on PE2. Without a matching RT, PE1 does not consider the VPNv4 route as belonging to the CUST VRF, so it cannot resolve the next hop or assign a label for forwarding.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
MTU mismatch on the MPLS core interfaces
Why it's wrong here
MTU issues cause packet drops, not missing label entries.
- ✗
LDP is not enabled on the core link between PE1 and P
Why it's wrong here
LDP is needed for transport labels, but the issue is about VPN label in forwarding table.
- ✓
Route target mismatch between PE1 and PE2
Why this is correct
Incorrect RT prevents the VPN label from being installed in the forwarding table despite BGP advertisement.
- ✗
BGP session between PE1 and PE2 is not established
Why it's wrong here
If BGP session is down, the route would not appear in 'show ip bgp vpnv4'.
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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.