The answer is that R1 is configured for label distribution on-demand mode. This is correct because in on-demand mode, as defined in RFC 5036, a router only requests label bindings from an LDP neighbor for prefixes that are actively in its own routing table or that it needs to forward traffic toward. Even though the LDP session is up and the neighbor relationship is established, R1 will not request or install label bindings for prefixes learned from R2 unless R1 itself has those exact prefixes in its routing table and requires a label to reach them. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of the difference between on-demand and liberal label distribution modes—a common trap is assuming a fully established LDP session guarantees label bindings for all prefixes. Remember that on-demand mode is stingy: it only asks for labels when it needs them, so an empty forwarding table with a live neighbor is a dead giveaway. Memory tip: “On-demand means only demand what you need—no free labels.”
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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
R1# show mpls ldp neighbor
Peer LDP Ident: 10.0.0.2:0; Local LDP Ident: 10.0.0.1:0
TCP connection: 10.0.0.2.646 - 10.0.0.1.54658
State: Oper; Msgs sent/rcvd: 100/100; Downstream on demand
Up time: 01:23:45
LDP discovery sources:
GigabitEthernet0/0/0; hello interval: 5 s, hold time: 15 s
Addresses bound to peer LDP Ident:
10.0.0.2 10.1.1.2
Refer to the exhibit. An engineer notices that R1 has an LDP neighbor but 'show mpls forwarding-table' on R1 shows no label bindings for prefixes learned from R2. What 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
✓
R1 is configured for 'label distribution on-demand'
R1 shows an LDP neighbor (session is up) but no label bindings for prefixes from R2. This occurs when R1 is configured for 'label distribution on-demand' (RFC 5036), meaning it only requests label bindings for prefixes in its routing table, not all prefixes from the peer. Since R1 has not yet needed those specific prefixes, it has not requested labels, so the forwarding table remains empty.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 discovery source is incorrect
Why it's wrong here
Discovery source shows correct interface.
✗
The peer LDP identifier is not reachable
Why it's wrong here
Identifiers are IP addresses, reachable since session is up.
✗
LDP session is not established
Why it's wrong here
State is Oper, so session is up.
✓
R1 is configured for 'label distribution on-demand'
Why this is correct
Downstream on-demand means labels are not sent until requested; if no request, no labels.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between LDP session state (neighbor adjacency) and label binding exchange, tricking candidates into thinking a working session guarantees label bindings, when in fact 'label distribution on-demand' can suppress label advertisements.
Trap categories for this question
Command / output trap
Discovery source shows correct interface.
Detailed technical explanation
How to think about this question
In default LDP operation (liberal label distribution), a router advertises labels for all prefixes in its routing table to all peers. With 'label distribution on-demand' (configured via 'mpls ldp label distribution on-demand' under the LDP router or interface), the router only requests labels for prefixes it actively uses, reducing control-plane overhead in large networks. This can cause forwarding-table entries to be missing until traffic triggers a label request, which is a common optimization in service provider cores with many VPNs.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
MPLS and Segment Routing — This question tests MPLS and Segment Routing — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: R1 is configured for 'label distribution on-demand' — R1 shows an LDP neighbor (session is up) but no label bindings for prefixes from R2. This occurs when R1 is configured for 'label distribution on-demand' (RFC 5036), meaning it only requests label bindings for prefixes in its routing table, not all prefixes from the peer. Since R1 has not yet needed those specific prefixes, it has not requested labels, so the forwarding table remains empty.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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?
Read the scenario before looking for a memorised answer.
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