The answer is that the route to 10.4.4.0/24 is not the best path in the routing table. This is the most likely cause because LDP only assigns and advertises labels for routes that are actually used for forwarding, meaning the route must be the best path based on the routing table’s selection criteria, such as administrative distance or IGP metric. If a different path to 10.4.4.0/24 is preferred, the router will not generate a local label binding for that prefix, even though the LDP neighbor session is up and the prefix exists in the routing table. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the fundamental LDP label advertisement rule: labels are only bound to the best, active route. A common trap is to assume a working LDP session guarantees label advertisement, but the session only enables label exchange—it does not override routing table decisions. Memory tip: LDP labels follow the best path, not just any path.
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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
Router# show mpls ldp neighbor
Peer LDP Ident: 10.1.1.2:0; Local LDP Ident: 10.1.1.1:0
TCP connection: 10.1.1.2.646 - 10.1.1.1.35399
State: Oper; Msgs sent/rcvd: 148/143; Downstream
Up time: 02:15:10
LDP discovery sources:
GigabitEthernet0/0/0; Src IP addr: 10.1.1.2
Addresses bound to peer LDP Ident:
10.1.1.2 10.2.2.2
Peer LDP Ident: 10.2.2.2:0; Local LDP Ident: 10.1.1.1:0
TCP connection: 10.2.2.2.646 - 10.1.1.1.49201
State: Oper; Msgs sent/rcvd: 73/67; Downstream
Up time: 01:10:45
LDP discovery sources:
GigabitEthernet0/0/1; Src IP addr: 10.2.2.2
Addresses bound to peer LDP Ident:
10.2.2.2 10.3.3.3
Peer LDP Ident: 10.3.3.3:0; Local LDP Ident: 10.1.1.1:0
TCP connection: 10.3.3.3.646 - 10.1.1.1.52001
State: Oper; Msgs sent/rcvd: 53/49; Downstream
Up time: 00:45:30
LDP discovery sources:
GigabitEthernet0/0/2; Src IP addr: 10.3.3.3
Addresses bound to peer LDP Ident:
10.3.3.3 10.4.4.4
Refer to the exhibit. The router has three LDP neighbors established. A network administrator notices that MPLS labels for the prefix 10.4.4.0/24 are not being advertised from this router to its neighbors, although the prefix is present in the routing table. Which is the most likely 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 route to 10.4.4.0/24 is not the best path in the routing table.
Option B is correct because the LDP neighbor for 10.4.4.4 (via 10.3.3.3) is up, but the router does not have a label for 10.4.4.0/24. This suggests the route is not being used for forwarding because of the IGP metric or administrative distance. Option A is wrong because the neighbor is up. Option C is wrong because LDP has been configured globally. Option D is wrong because there is no indication of an mroute issue.
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 LDP session to 10.3.3.3 is down.
Why it's wrong here
The session is in Oper state, so it is up.
✓
The route to 10.4.4.0/24 is not the best path in the routing table.
Why this is correct
LDP assigns a label only to the best IGP route; if the route is not best, no label is allocated.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The 'mpls ldp' command has not been applied globally.
Why it's wrong here
LDP peers are established, so it must be configured globally.
✗
The multicast routing table is empty for the prefix.
Why it's wrong here
Multicast routing is unrelated to LDP label distribution.
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.
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 route to 10.4.4.0/24 is not the best path in the routing table. — Option B is correct because the LDP neighbor for 10.4.4.4 (via 10.3.3.3) is up, but the router does not have a label for 10.4.4.0/24. This suggests the route is not being used for forwarding because of the IGP metric or administrative distance. Option A is wrong because the neighbor is up. Option C is wrong because LDP has been configured globally. Option D is wrong because there is no indication of an mroute issue.
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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