Quick Answer
The correct choice is multiple parallel links with ECMP routing because this design directly addresses the need to minimize latency and maximize bandwidth for leaf-spine east-west traffic. In a leaf-spine topology, server-to-server traffic must traverse the spine layer, and ECMP allows all parallel links to remain active simultaneously, load-balancing packets across equal-cost paths using Layer 3 protocols like OSPF or BGP. This eliminates the bottleneck of a single link and ensures non-blocking, high-throughput data flow. On the Cisco DevNet Associate 200-901 exam, this concept tests your understanding of data center fabric design and the role of ECMP in achieving efficient east-west traffic distribution—a common trap is assuming Spanning Tree Protocol (STP) is used, but STP blocks redundant links, whereas ECMP actively uses them. Memory tip: think “ECMP = Every Core Multi-Path” to recall that all parallel links stay active for east-west traffic.
This 200-901 practice question tests your understanding of network fundamentals. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 network engineer is designing a data center network with leaf-spine topology. The requirement is to minimize latency and maximize bandwidth for east-west traffic. Which type of links should be used between leaf and spine switches?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "minimum / minimize"
Why it matters: Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Multiple links with VSS
Why wrong: VSS is proprietary and not standard for leaf-spine.
Single link with LACP
Why wrong: LACP bundles ports but still single logical link.
Multiple parallel links with ECMP routing
ECMP allows all links to be active, increasing bandwidth and reducing latency.
Single link with STP
Why wrong: STP blocks redundant links, reducing bandwidth.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Multiple parallel links with ECMP routing
In a leaf-spine topology, east-west traffic (server-to-server) must traverse the spine switches. Using multiple parallel links with Equal-Cost Multi-Path (ECMP) routing allows all links to be active simultaneously, maximizing bandwidth and minimizing latency by load-balancing traffic across all available paths. ECMP leverages Layer 3 routing (e.g., OSPF or BGP) to forward packets over multiple equal-cost paths, which is ideal for the non-blocking, high-throughput design of leaf-spine architectures.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Multiple links with VSS
Why it's wrong here
VSS is proprietary and not standard for leaf-spine.
Single link with LACP
Why it's wrong here
LACP bundles ports but still single logical link.
Multiple parallel links with ECMP routing
Why this is correct
ECMP allows all links to be active, increasing bandwidth and reducing latency.
Clue confirmation
The clue word "minimum / minimize" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Single link with STP
Why it's wrong here
STP blocks redundant links, reducing bandwidth.
Common exam traps
Cisco often tests the misconception that link aggregation (LACP or VSS) is the best way to increase bandwidth in a leaf-spine design, but the trap is that these are Layer 2 solutions that do not provide the active-active multipath routing (ECMP) required for optimal east-west traffic in a Layer 3 leaf-spine topology.
Detailed technical explanation
ECMP in a leaf-spine network typically uses a hash-based load-balancing algorithm (e.g., based on source/destination IP, port, or flow) to distribute traffic across multiple equal-cost Layer 3 paths. Under the hood, each leaf switch has multiple routes to the same destination via different spine switches, and the forwarding table installs all of them; the switch then selects a path per flow using a hash to avoid packet reordering. In real-world deployments, ECMP with BGP (RFC 4271) or OSPF (RFC 2328) is common, and the number of parallel links can be scaled to achieve full line-rate throughput, making it the standard for modern data center fabrics like Cisco ACI or VXLAN EVPN.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Network Fundamentals — This question tests Network Fundamentals — Read the scenario before looking for a memorised answer..
The correct answer is: Multiple parallel links with ECMP routing — In a leaf-spine topology, east-west traffic (server-to-server) must traverse the spine switches. Using multiple parallel links with Equal-Cost Multi-Path (ECMP) routing allows all links to be active simultaneously, maximizing bandwidth and minimizing latency by load-balancing traffic across all available paths. ECMP leverages Layer 3 routing (e.g., OSPF or BGP) to forward packets over multiple equal-cost paths, which is ideal for the non-blocking, high-throughput design of leaf-spine architectures.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "minimum / minimize". Asks for the least resource use — fewest addresses, smallest subnet, lowest overhead. Eliminate over-provisioned options even if they would technically work.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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