The answer is to enable per-flow load balancing with the 'load-balance per-flow' command on all routers. This is correct because in an MPLS L3VPN, when the IGP shows equal-cost paths but the MPLS forwarding table on the P router has only one label entry for the BGP next-hop, the default CEF behavior often fails to hash traffic across multiple LSPs, causing all traffic to pin to a single path. Enabling per-flow load balancing forces the router to use a hash of the IP header fields (source/destination IP, ports) to distribute flows across the equal-cost paths, fixing the one-path issue. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of MPLS load-balancing mechanics and the distinction between per-packet and per-flow modes—a common trap is assuming IGP equal-cost routes automatically guarantee MPLS load sharing. Memory tip: think “per-flow fixes the flow—one path is no-go.”
350-501 Architecture Practice Question
This 350-501 practice question tests your understanding of architecture. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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'
Option B is correct because 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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
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.
Common exam traps
Common exam trap: answer the scenario, not the keyword
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.
Detailed technical explanation
How to think about this question
In MPLS networks, load balancing across equal-cost paths requires per-flow hashing at the ingress and transit routers to distribute traffic based on packet headers (e.g., IP addresses, ports). The default CEF load-balancing mode is often per-destination, which can lead to all traffic using a single path if the BGP next-hop is the same; enabling 'load-balance per-flow' changes the hash algorithm to consider Layer 3 and Layer 4 information, ensuring better distribution. In real-world deployments, this is critical for maximizing link utilization in MPLS L3VPN services where customer traffic may have many flows but a single next-hop IP.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Architecture — This question tests Architecture — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable per-flow load balancing on all routers with 'load-balance per-flow' — Option B is correct because 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.
What should I do if I get this 350-501 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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