350-501 MPLS and Segment Routing Practice Question
Exhibit
Refer to the exhibit. The configuration snippet is from a PE router in an MPLS L3VPN setup. ip vrf CUSTOMER rd 100:1 route-target export 100:1 route-target import 100:1 ! interface GigabitEthernet0/0 ip vrf forwarding CUSTOMER ip address 192.168.1.1 255.255.255.252 ! interface GigabitEthernet0/1 ip address 10.0.0.1 255.255.255.252 mpls ip ! router bgp 100 bgp log-neighbor-changes neighbor 10.0.0.2 remote-as 100 neighbor 10.0.0.2 update-source Loopback0 ! address-family vpnv4 neighbor 10.0.0.2 activate neighbor 10.0.0.2 send-community extended ! address-family ipv4 vrf CUSTOMER redistribute connected !
The CE router behind the VRF interface is unable to reach the remote CE. Which configuration error is most likely the cause?
⚠ Common exam trap
Cisco often tests the subtle interaction between BGP update-source and neighbor address configuration in MPLS VPN scenarios, where candidates mistakenly think the directly connected interface address is always sufficient for BGP peering, ignoring that the update-source overrides the source IP and must be routable.
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 neighbor is configured with a directly connected interface address but uses update-source Loopback0.
When BGP is configured with a neighbor using a directly connected interface address (e.g., 10.0.0.2) but the update-source is set to Loopback0, the BGP session will attempt to establish using the loopback IP as the source. If the loopback address is not reachable via the directly connected link (e.g., no IGP route for the loopback across the MPLS/VPN core), the BGP session will fail, preventing the CE router behind the VRF from reaching the remote CE. This mismatch between the neighbor address and the update-source is a common misconfiguration in MPLS L3VPN environments.
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 BGP neighbor is configured with a directly connected interface address but uses update-source Loopback0.
Why this is correct
This mismatch causes BGP to try to source from the loopback while expecting a session to the interface address, likely leading to session failure.
- ✗
The VRF is missing the 'rd' command.
Why it's wrong here
The 'rd' is present (100:1).
- ✗
The 'mpls ip' command is missing on the core interface.
Why it's wrong here
The command is present on Gi0/1.
- ✗
The VRF interface does not have an IP address.
Why it's wrong here
It has IP address 192.168.1.1/30.
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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.