350-501 Networking Practice Question
A network engineer is troubleshooting MPLS LSP connectivity. The MPLS LDP session is up on both endpoints, but some MPLS labels are missing in the LFIB. Which configuration change would most likely resolve the issue?
⚠ Common exam trap
Cisco often tests the distinction between LDP session establishment (which can be up) and label advertisement (which can be suppressed via filters), leading candidates to overlook the 'advertise-labels' command and instead focus on interface-level MPLS enablement or protocol selection.
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
✓
Enable 'mpls ldp advertise-labels'
The 'mpls ldp advertise-labels' command ensures that LDP advertises label bindings to its peers. If this is disabled (default is enabled, but can be overridden), LDP sessions may be up but no labels are distributed, resulting in missing labels in the LFIB. Re-enabling it forces LDP to send label mappings for all FECs, resolving the connectivity issue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Configure 'label protocol ldp' on all routers
Why it's wrong here
LDP is usually the default label protocol.
- ✓
Enable 'mpls ldp advertise-labels'
Why this is correct
This command ensures that LDP advertises labels for all FECs.
- ✗
Configure 'mpls ip' on all interfaces
Why it's wrong here
Missing labels are not typically caused by missing 'mpls ip' if LDP is up.
- ✗
Enable 'mpls ldp explicit-null' on the ingress
Why it's wrong here
Enabling `mpls ldp explicit-null` on the ingress forces the imposition of an explicit-null label (value 0) for directly connected next-hops, but the scenario describes missing labels in the LFIB, which is a forwarding-plane table populated by label bindings from the LDP peer. Explicit-null does not generate new label bindings or repair missing LFIB entries; it only alters the label value imposed for penultimate-hop popping. It is tempting because explicit-null is often used to preserve QoS or ECN markings across the MPLS domain, but that use case addresses label-stack behaviour, not the absence of label mappings in the LFIB.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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