easyMultiple Choice
300-410 Practice Question: The default behavior of LDP when establishing a…
What is the default behavior of LDP when establishing a session between two directly connected routers?
⚠ Common exam trap
Candidates often confuse LDP's Hello multicast address (224.0.0.2) with OSPF's 224.0.0.5, or assuming LDP uses a single transport protocol for both discovery and session establishment.
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
✓
LDP uses TCP port 646 for session establishment and UDP port 646 for hellos.
LDP uses UDP port 646 for the initial Hello discovery messages (sent to multicast 224.0.0.2) and TCP port 646 for the reliable session establishment and label advertisement exchange. This dual-transport design is fundamental to how LDP separates neighbor discovery from session maintenance.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
LDP sends hellos to the unicast address of each neighbor.
Why it's wrong here
LDP hellos are sent to the multicast address 224.0.0.2, not to each neighbour's unicast address; unicast is used only for the subsequent TCP session on port 646. Unicast targeting is tempting because it mirrors BGP's explicitly configured neighbour statements, but LDP relies on multicast discovery before session establishment.
- ✓
LDP uses TCP port 646 for session establishment and UDP port 646 for hellos.
Why this is correct
LDP satisfies the directly connected constraint by separating discovery from session setup: hellos use UDP port 646 to find link-local peers, then TCP port 646 establishes the session. This matches the stem's requirement for directly connected routers, where UDP hellos are link-scoped and TCP provides reliable session transport.
- ✗
LDP hellos are sent to the all-OSPF-routers multicast address 224.0.0.5.
Why it's wrong here
LDP discovers neighbours by sending hellos to the link-local multicast address 224.0.0.2 on UDP port 646, not the OSPF all-routers address 224.0.0.5, which carries OSPF hello packets. The OSPF multicast address is tempting because both protocols use hellos for adjacency, but LDP's discovery mechanism is protocol-independent.
- ✗
LDP sessions are established using UDP for reliability.
Why it's wrong here
LDP sessions use TCP for reliable transport; UDP is only for hellos.
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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