The correct behavior is that the router installs the received label as the incoming label for the prefix. This occurs because when a router has a route to that prefix in its routing table, it uses the label mapping received from its neighbor to populate its Label Information Base (LIB) as the incoming label, enabling label switching for that FEC. On the Cisco SPCOR 350-501 exam, this tests your understanding of MPLS LDP label mapping installation behavior, specifically the distinction between incoming and outgoing label roles. A common trap is confusing the incoming label (received from a neighbor) with the outgoing label (which the router assigns and advertises downstream); remember that the received label becomes the local incoming label for traffic entering the LSP. A useful memory tip is “Receive In, Send Out”—the label you receive is for inbound traffic, while the label you assign is for outbound traffic.
350-501 MPLS and Segment Routing Practice Question
This 350-501 practice question tests your understanding of mpls and segment routing. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
During MPLS LDP operation, a router receives a label mapping for a prefix from its neighbor. What is the correct behavior when the receiving router has a route to that prefix?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It installs the label as the incoming label for the prefix.
The router installs the received label as the incoming label for that prefix in its LIB and switches to that label. Option A is incorrect because it does not install an outgoing label; it stores the mapping. Option C is incorrect because it does not discard; it uses it. Option D is incorrect because LDP uses UDP for discovery, but TCP for session; the label mapping is over TCP.
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.
✗
It installs the label as the outgoing label for the prefix.
Why it's wrong here
The received label is the incoming label from the neighbor; it becomes the outgoing label for traffic destined to that prefix.
✗
It discards the label mapping unless it has a corresponding route.
Why it's wrong here
The router accepts the mapping and stores it if it has a route; if not, it may store it for later.
✗
It uses UDP to send a notification back.
Why it's wrong here
LDP uses TCP for reliable session; no UDP notification.
✓
It installs the label as the incoming label for the prefix.
Why this is correct
The router updates its LIB with the received label as the outgoing label for the prefix.
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: It installs the label as the incoming label for the prefix. — The router installs the received label as the incoming label for that prefix in its LIB and switches to that label. Option A is incorrect because it does not install an outgoing label; it stores the mapping. Option C is incorrect because it does not discard; it uses it. Option D is incorrect because LDP uses UDP for discovery, but TCP for session; the label mapping is over TCP.
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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