350-501 MPLS and Segment Routing Practice Question
A network engineer is troubleshooting an LDP session failure between two directly connected routers. The routers are configured with the 'mpls ldp' command under the interface. The 'show mpls ldp neighbor' command shows no neighbors. Which two additional pieces of information should the engineer verify? (Choose two.)
⚠ Common exam trap
Cisco often tests the distinction between LDP Hello (UDP multicast) and LDP session (TCP unicast), and candidates mistakenly focus on TCP ACLs or global MPLS commands instead of verifying subnet adjacency and router ID reachability.
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
✓
Verify that the interface IP addresses are in the same subnet.
LDP sessions are established only between directly connected LSRs when using the default 'mpls ldp' interface configuration. The LDP Hello messages are sent as UDP packets to the multicast address 224.0.0.2, and if the interface IP addresses are not in the same subnet, the Hello messages will not be received, preventing neighbor discovery.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Verify that the interface IP addresses are in the same subnet.
Why this is correct
LDP hello messages are sent to the all-routers multicast address; mismatched subnets prevent discovery.
- ✗
Verify that TCP port 646 is open on the interface ACL.
Why it's wrong here
LDP uses UDP for discovery and TCP for session; but the question asks for verification steps, and this is too specific; not the first step.
- ✗
Verify that 'mpls ip' is enabled globally.
Why it's wrong here
The 'mpls ip' command is not needed; 'mpls ldp' under interface is sufficient.
- ✓
Verify that the LDP router IDs are reachable via routing.
Why this is correct
LDP uses TCP to establish session; the transport address must be reachable.
- ✗
Verify that OSPF is configured on the interface.
Why it's wrong here
LDP does not require OSPF; any IGP works.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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