350-501 Networking Practice Question
Exhibit
Router# show mpls ldp neighbor
Peer LDP Ident: 10.1.1.2:0; Local LDP Ident: 10.1.1.1:0
TCP connection: 10.1.1.2.646 - 10.1.1.1.52300
State: Oper; Msgs sent/rcvd: 100/101; Downstream on demand
Up time: 01:23:45
LDP discovery sources:
GigabitEthernet0/0/0; xc: 10.1.1.2:0
Addresses bound to peer LDP Ident:
10.2.2.2 10.3.3.3Refer to the exhibit. An engineer is troubleshooting an MPLS LDP session between two routers. The output shows that the LDP session is operational. However, MPLS labels are not being exchanged. What is the most likely cause?
⚠ Common exam trap
The trap here is that candidates see the LDP session is 'Operational' and assume label exchange must be working, but Cisco tests the subtle distinction that session state and label advertisement are independent processes.
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
✓
The LDP label advertisement mode is 'downstream on demand'.
The exhibit shows that the LDP session is operational (state = Operational), but no labels are being exchanged. In 'downstream on demand' mode, a router does not advertise labels unless explicitly requested by an upstream neighbor via a Label Request message. Since the session is up but no labels are exchanged, this mismatch in label advertisement mode is the most likely cause.
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 peer's addresses are not bound to the LDP Ident.
Why it's wrong here
Addresses are bound as shown.
- ✗
The TCP connection is not using port 646.
Why it's wrong here
The output shows port 646 is used.
- ✓
The LDP label advertisement mode is 'downstream on demand'.
Why this is correct
In this mode, labels are only sent on request; if the peer expects unsolicited, labels may not be exchanged.
- ✗
The LDP session is not established.
Why it's wrong here
The session state is Oper, meaning operational.
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