350-501 LDP Label Mapping Practice Question
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?
⚠ Common exam trap
Cisco often tests the distinction between incoming and outgoing labels in LDP, trapping candidates who assume the received label is installed as the incoming label. The received label is actually the outgoing label for forwarding.
Answer choices
Why each option matters
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 outgoing label for the prefix.
When a router receives a label mapping from an LDP neighbor for a prefix, it installs the received label as the outgoing label for that prefix in its Label Forwarding Information Base (LFIB). This label is used when forwarding packets toward the neighbor. The router must have a route to the prefix in its routing table to accept the mapping.
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 this is correct
Correct. The router installs the received label as the outgoing label for the prefix, because the label is from the next-hop neighbor and is used to forward packets toward that neighbor.
- ✗
It discards the label mapping unless it has a corresponding route.
Why it's wrong here
Incorrect. The router must have a route to the prefix to accept the label mapping, but it does not discard it; it installs it as the outgoing label.
- ✗
It uses UDP to send a notification back.
Why it's wrong here
Incorrect. LDP uses TCP for session establishment and notification, not UDP.
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It installs the label as the incoming label for the prefix.
Why it's wrong here
Incorrect. The received label is not installed as the incoming label; the incoming label is assigned locally and advertised to neighbors.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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