350-501 MPLS and Segment Routing Practice Question
Exhibit
R1# show mpls ldp neighbor
Peer LDP Ident: 10.0.0.2:0; Local LDP Ident: 10.0.0.1:0
TCP connection: 10.0.0.2.646 - 10.0.0.1.54658
State: Oper; Msgs sent/rcvd: 100/100; Downstream on demand
Up time: 01:23:45
LDP discovery sources:
GigabitEthernet0/0/0; hello interval: 5 s, hold time: 15 s
Addresses bound to peer LDP Ident:
10.0.0.2 10.1.1.2Refer to the exhibit. An engineer notices that R1 has an LDP neighbor but 'show mpls forwarding-table' on R1 shows no label bindings for prefixes learned from R2. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the distinction between LDP session state (neighbor adjacency) and label binding exchange, tricking candidates into thinking a working session guarantees label bindings, when in fact 'label distribution on-demand' can suppress label advertisements.
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
✓
R1 is configured for 'label distribution on-demand'
R1 shows an LDP neighbor (session is up) but no label bindings for prefixes from R2. This occurs when R1 is configured for 'label distribution on-demand' (RFC 5036), meaning it only requests label bindings for prefixes in its routing table, not all prefixes from the peer. Since R1 has not yet needed those specific prefixes, it has not requested labels, so the forwarding table remains empty.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The LDP discovery source is incorrect
Why it's wrong here
Discovery source shows correct interface.
- ✗
The peer LDP identifier is not reachable
Why it's wrong here
Identifiers are IP addresses, reachable since session is up.
- ✗
LDP session is not established
Why it's wrong here
State is Oper, so session is up.
- ✓
R1 is configured for 'label distribution on-demand'
Why this is correct
Downstream on-demand means labels are not sent until requested; if no request, no labels.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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