350-501 Architecture Practice Question
A service provider is deploying an MPLS L3VPN solution for a customer with two sites. The PE routers are running OSPF as the IGP and LDP for label distribution. The provider wants to ensure that customer traffic is load-balanced across two equal-cost paths between the PEs. The network team notices that all traffic is taking only one path despite the IGP showing equal-cost routes. The team has verified that the MPLS forwarding table on the P router shows only one label entry for the BGP next-hop. The team suspects a load-balancing issue. Which action best resolves the problem?
⚠ Common exam trap
Cisco often tests the misconception that IGP equal-cost routes automatically guarantee MPLS load balancing, but the trap here is that MPLS forwarding uses the label entry for the BGP next-hop, and without per-flow load balancing, the router may install only one label entry in the LFIB (Label Forwarding Information Base) for that next-hop, causing all traffic to follow a single path.
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 per-flow load balancing on all routers with 'load-balance per-flow'
The issue is that MPLS L3VPN traffic is not being load-balanced despite equal-cost IGP paths. This typically occurs when the MPLS forwarding table on the P router has only one label entry for the BGP next-hop, indicating that per-flow load balancing is not enabled. Enabling 'load-balance per-flow' on all routers ensures that the CEF (Cisco Express Forwarding) uses per-flow hashing to distribute traffic across multiple equal-cost LSPs (Label Switched Paths), rather than per-packet or default behavior that may favor a single path.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the OSPF maximum-paths value to 4
Why it's wrong here
The IGP already has equal-cost paths; increasing maximum-paths does not help if load balancing is not enabled at the MPLS level.
- ✓
Enable per-flow load balancing on all routers with 'load-balance per-flow'
Why this is correct
This enables load balancing based on flow, which is required for MPLS to use multiple equal-cost paths.
- ✗
Enable BGP additional-paths on the PE routers
Why it's wrong here
Additional-paths allows BGP to advertise multiple paths, but the issue is with load balancing in the data plane, not control plane.
- ✗
Configure LDP to use the 'mpls ldp igp sync' command
Why it's wrong here
IGP sync ensures LDP labels are distributed before the IGP route is used, but does not affect load balancing.
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JA
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.