The answer is UDP for discovery and TCP for session. MPLS LDP uses UDP hello messages on port 646 to discover directly connected neighbors, then establishes a TCP session on the same port to reliably exchange label bindings for prefixes found in the IGP routing table. This separation ensures fast neighbor discovery without session overhead, while TCP guarantees ordered delivery of label mappings. On the Cisco SPCOR 350-501 exam, this distinction is frequently tested as a foundational concept, with common traps including confusion over LDP’s inability to distribute labels for BGP routes or the mistaken belief that LDP uses a single transport protocol. Remember the mnemonic: “Discover with UDP, Discuss with TCP” to lock in the pairing.
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. 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.
Which TWO of the following are characteristics of MPLS LDP?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Label distribution based on IP routing table
B is correct because MPLS LDP distributes labels for routes found in the IP routing table, not for BGP routes. LDP peers exchange label bindings for each prefix in the IGP routing table, enabling label-switched paths (LSPs) for those destinations. This is fundamental to LDP's operation as defined in RFC 5036.
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.
✗
Distributes labels for BGP routes
Why it's wrong here
LDP does not distribute labels for BGP routes unless specifically configured.
✓
Label distribution based on IP routing table
Why this is correct
By default, LDP distributes labels for all IGP routes.
Related concept
Read the scenario before looking for a memorised answer.
✗
Requires an IGP like OSPF for session reachability
Why it's wrong here
LDP session can be established over any reachable IP, not solely IGP.
✓
UDP for discovery, TCP for session
Why this is correct
LDP uses UDP hellos and TCP for session establishment.
Related concept
Read the scenario before looking for a memorised answer.
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that LDP requires an IGP for session reachability, but the trap is that LDP uses its own UDP discovery mechanism and can form sessions over any IP reachable path, though an IGP is commonly used for loopback reachability in practice.
Detailed technical explanation
How to think about this question
LDP uses UDP multicast hellos (224.0.0.2) to discover directly connected neighbors, then establishes a TCP session for reliable label exchange. The label distribution is driven by the IP routing table, meaning LDP assigns a label for every IGP prefix (e.g., OSPF, IS-IS) in the FIB, but not for BGP routes unless explicitly configured with 'label bgp' or similar. In real-world MPLS deployments, LDP is often used for transport LSPs, while BGP handles VPN labels, creating a two-label stack.
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.
Networking — This question tests Networking — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Label distribution based on IP routing table — B is correct because MPLS LDP distributes labels for routes found in the IP routing table, not for BGP routes. LDP peers exchange label bindings for each prefix in the IGP routing table, enabling label-switched paths (LSPs) for those destinations. This is fundamental to LDP's operation as defined in RFC 5036.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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