350-501 Architecture Practice Question
Which protocol is used to discover MPLS peers for LDP session establishment?
⚠ Common exam trap
Cisco often tests the misconception that LDP uses TCP for discovery because TCP is used for the subsequent label exchange, but the initial peer discovery is explicitly UDP-based.
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
✓
UDP
LDP uses UDP multicast (224.0.0.2) on port 646 to discover directly connected MPLS peers, as specified in RFC 5036. This allows LDP to dynamically find neighbors without manual configuration, using UDP for the initial discovery phase before establishing a TCP session for label exchange.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
TCP
Why it's wrong here
TCP is used for the LDP session after discovery.
- ✓
UDP
Why this is correct
LDP hello messages are sent via UDP on port 646.
- ✗
ARP
Why it's wrong here
ARP resolves IP to MAC addresses.
- ✗
ICMP
Why it's wrong here
ICMP is for control messages, not discovery.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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