The correct answer is that downstream label mode means labels are advertised to all neighbors without a request. This is because in LDP downstream unsolicited mode, a router proactively distributes label bindings for all its prefixes to every LDP neighbor, regardless of whether the neighbor has asked for them. On the Cisco SPCOR 350-501 exam, this concept tests your understanding of the fundamental difference between downstream unsolicited (DU) and downstream-on-demand (DoD) label distribution modes, often appearing in exhibit-based questions where show commands reveal the label advertisement method. A common trap is confusing "downstream" with "downstream-on-demand"—remember that downstream alone implies unsolicited, push-based advertising, while DoD requires a request. For a quick memory tip, think of "Downstream Unsolicited" as "DU" for "Deliver Unasked," meaning the router sends labels without waiting for a query.
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
Refer to the exhibit.
Router# show mpls ldp neighbor
Peer LDP Ident: 192.168.1.2:0; Local LDP Ident: 192.168.1.1:0
TCP connection: 192.168.1.2.646 - 192.168.1.1.58370
State: Operacional; Msgs sent/rcvd: 342/341; Downstream
Up time: 02:34:21
LDP discovery sources:
GigabitEthernet0/0/0/0; Src IP addr: 192.168.1.2, transport address: 192.168.1.2
targeted-hello 192.168.1.2 -> 192.168.1.1
holdtime: 15 sec; proposed local/peer: 15/15
LDP id: 192.168.1.2:0
Refer to the exhibit. The show command output displays the LDP neighbor state. What does the 'Downstream' label mode indicate about label distribution?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Labels are advertised to all neighbors without a request.
Downstream label mode means the router advertises labels to its neighbors without being asked. Option A is incorrect because downstream-on-demand sends labels only when requested. Option C is incorrect because unsolicited downstream is the same as downstream. Option D is incorrect because DoD is not shown.
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.
✗
Labels are sent only to the peer that advertises the prefix.
Why it's wrong here
Downstream mode sends to all neighbors regardless of request.
✗
Labels are only distributed when explicitly requested.
Why it's wrong here
That describes Downstream-on-Demand.
✗
The neighbor must request labels via a Label Request message.
Why it's wrong here
That is downstream-on-demand.
✓
Labels are advertised to all neighbors without a request.
Why this is correct
Downstream mode means unsolicited label advertisement.
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: Labels are advertised to all neighbors without a request. — Downstream label mode means the router advertises labels to its neighbors without being asked. Option A is incorrect because downstream-on-demand sends labels only when requested. Option C is incorrect because unsolicited downstream is the same as downstream. Option D is incorrect because DoD is not shown.
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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