350-501 MPLS and Segment Routing Practice Question
A network engineer notices that BGP-LU prefixes learned from an eBGP neighbor are not being installed in the MPLS forwarding table (LFIB). The BGP table shows the prefixes with the correct label. The IGP route to the neighbor is reachable. What additional configuration is needed?
⚠ Common exam trap
Cisco often tests the distinction between BGP label allocation modes (per-prefix vs. per-next-hop) and the requirement for per-prefix mode to install BGP-LU prefixes into the LFIB, confusing candidates who assume any BGP label automatically populates the MPLS forwarding table.
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
✓
Set `label mode per-prefix` under the BGP neighbor or address-family
BGP-LU (BGP Labeled Unicast) installs labels into the BGP table, but for those labels to be used for MPLS forwarding, the router must be configured to allocate labels on a per-prefix basis. By default, BGP may use per-VRF or per-next-hop label allocation, which does not populate the LFIB with per-prefix entries. Setting `label mode per-prefix` under the BGP neighbor or address-family forces BGP to allocate a unique label for each prefix, enabling the LFIB installation required for MPLS forwarding.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Enable `no bgp default ipv4-unicast`
Why it's wrong here
That command prevents IPv4 unicast from being advertised, not label installation.
- ✗
Configure `mpls ldp autoconfig` on the interface
Why it's wrong here
LDP is not needed for BGP-LU labels.
- ✗
Add `allowas-in` to accept routes with AS-path containing own AS
Why it's wrong here
allowas-in deals with AS-path loop prevention, not label installation.
- ✓
Set `label mode per-prefix` under the BGP neighbor or address-family
Why this is correct
BGP-LU requires per-prefix label allocation for LFIB installation.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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