350-401 Architecture Practice Question
A network engineer is designing a campus network and needs to ensure high availability for the core layer. Which design best practice should be implemented?
⚠ Common exam trap
Cisco often tests the misconception that the core layer should remain Layer 2 for simplicity, but in modern campus designs, the core must route at Layer 3 to avoid STP convergence delays and support ECMP load balancing.
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
✓
Deploy two core switches configured with VSS or StackWise.
Deploying two core switches with VSS (Virtual Switching System) or StackWise provides both redundancy and active-active load balancing at the core layer. VSS virtualizes two physical switches into a single logical switch, eliminating the need for Spanning Tree Protocol (STP) on inter-switch links and enabling sub-second failover. This design ensures high availability by removing single points of failure and maximizing throughput between distribution and core layers.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use a single distribution switch to simplify management.
Why it's wrong here
In a campus network, the distribution layer aggregates access switches and enforces policy, so a single distribution switch becomes a single point of failure. If that switch fails or reboots, every connected access switch loses connectivity to the rest of the network, directly violating high-availability requirements. A resilient design requires at least two distribution switches using HSRP/VRRP for first-hop gateway redundancy or stacking/VSS to provide active-active forwarding.
- ✓
Deploy two core switches configured with VSS or StackWise.
Why this is correct
VSS or StackWise combines two physical core switches into one logical device, providing stateful failover and sub-second convergence between the chassis. This approach enables active-active traffic forwarding and allows downstream switches to use Cross-Stack EtherChannel (MEC), so both links carry traffic rather than one being blocked by spanning tree. Because the pair appears as a single switch, routing protocols and STP see one node, which simplifies configuration and improves redundancy compared to a standalone pair running HSRP.
- ✗
Configure the core layer for Layer 2 switching only.
Why it's wrong here
A Layer 2-only core requires inter-VLAN routing to happen at the distribution or access layers, forcing traffic to take suboptimal paths and increasing latency. Since a Layer 2 core relies on spanning tree for loop prevention, one of any redundant uplinks is always blocked, wasting bandwidth and extending failover times when the active path fails. In modern campus network designs, the core should perform Layer 3 routing to provide equal-cost multipath load balancing, fast convergence via protocols like OSPF or BGP, and to contain broadcast domains.
- ✗
Use spanning-tree PortFast on all core switch ports.
Why it's wrong here
Spanning-tree PortFast bypasses STP listening/learning states for access ports to speed up host link-up, but it is not a redundancy feature. Applying PortFast to core ports connecting to other switches or the distribution layer removes spanning tree protections on those links, so if any redundant path exists, a temporary Layer 2 loop can form and cause a broadcast storm. The core should instead use STP features like LoopGuard or root guard, or better, use VSS/StackWise to avoid STP blocking altogether; PortFast alone never provides failover.
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Related to this question
Learn chapter
EtherChannel and Advanced Switching Technologies
Key term
High Availability in Enterprise Networks
High Availability in Enterprise Networks is the design principle that ensures network services remain accessible and operational with minimal downtime, even when individual components fail.
Key term
MSTP
MSTP (Multiple Spanning Tree Protocol) allows multiple VLANs to be grouped into a single spanning tree instance, reducing the number of spanning tree instances needed in a network.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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