The answer is a BGP neighbor update-source mismatch, where the neighbor is configured with a directly connected interface address but the update-source is set to Loopback0. This mismatch causes session failure because the router attempts to establish the TCP connection using the loopback address as the source, while the neighbor statement points to a directly connected IP that is not reachable from that loopback interface. On the Cisco SPCOR / CCNP Service Provider Core 350-501 exam, this scenario often appears in MPLS L3VPN troubleshooting questions, testing your understanding of how BGP peering works across VRF interfaces. A common trap is assuming any reachable IP will work, but the update-source must match the IP used in the neighbor statement. Remember the memory tip: “Peer IP and source IP must be the same address—if they differ, the session won’t ignite.”
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
The BGP neighbor is defined using the directly connected interface address 10.0.0.2, but the update-source is set to Loopback0. This mismatch prevents the BGP session from establishing because the router expects the neighbor to be reachable via the loopback. The neighbor should be the remote PE's loopback address.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Trap categories for this question
Command / output trap
The command is present on Gi0/1.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The BGP neighbor is configured with a directly connected interface address but uses update-source Loopback0. — The BGP neighbor is defined using the directly connected interface address 10.0.0.2, but the update-source is set to Loopback0. This mismatch prevents the BGP session from establishing because the router expects the neighbor to be reachable via the loopback. The neighbor should be the remote PE's loopback address.
What should I do if I get this 350-501 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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