350-501 Networking Practice Question
A service provider is deploying a new MPLS core network. The network has four routers: P1, P2, PE1, and PE2. OSPF is used as the IGP. The engineer configures MPLS LDP on all interfaces. After enabling LDP, the engineer notices that the LDP session between P1 and P2 is established, but no labels are exchanged for the loopback0 interfaces of PE1 and PE2. The loopback0 addresses are advertised in OSPF. The engineer verifies that the OSPF routes are present in the routing table of all routers. What is the most likely reason for the missing labels?
⚠ Common exam trap
Cisco often tests the misconception that 'mpls ip' must be enabled on the loopback interface itself for its prefix to receive a label, when in fact LDP advertises labels for any prefix in the routing table as long as the outgoing interface has 'mpls ip' enabled.
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
✓
An access-list is applied under 'mpls ldp advertise-labels' that denies the loopback prefixes.
The most likely reason is that an access-list is applied under 'mpls ldp advertise-labels' that denies the loopback prefixes. LDP by default advertises labels for all prefixes in the routing table, but the 'advertise-labels' command can filter which prefixes receive labels. If the loopback0 prefixes of PE1 and PE2 are denied by such an access-list, no labels will be advertised for them, even though OSPF routes are present.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
An access-list is applied under 'mpls ldp advertise-labels' that denies the loopback prefixes.
Why this is correct
Label advertisement can be filtered; this is a common issue.
- ✗
LDP is using UDP for label exchange.
Why it's wrong here
LDP uses TCP for label exchange.
- ✗
The OSPF cost to the loopbacks is too high.
Why it's wrong here
LDP does not consider IGP cost for label assignment.
- ✗
The loopback interfaces are not enabled with 'mpls ip'.
Why it's wrong here
'mpls ip' is needed on interfaces that LDP uses, but loopbacks are not typically used for LDP discovery.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-501 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-501 exam.