19 Oct 2026 · 6 min read · By Ezekiel T. Borbor

Redundant links are essential, but the mechanism that blocks loops at Layer 2 is not the same as the mechanism that chooses IP routes at Layer 3.

What Spanning Tree actually does

STP and its variants elect a loop-free forwarding topology for a bridged network. Depending on the design, a redundant switch link can be placed in a discarding or blocking state until the active topology changes.

What routing does

Routing uses a Layer 3 routing table to select next hops toward IP destinations. Static routing, dynamic protocols and first-hop redundancy mechanisms are separate from STP.

Practical verification

show spanning-tree vlan 10
show interfaces trunk
show ip route
show logging

Review the spanning-tree root, port roles and VLAN coverage. If routed interfaces are involved, verify the routes and adjacency independently.

Lab safety: Build the topology in a simulator first and verify failover behavior. Do not make production STP changes without an approved change plan.

Design takeaway

Document which technology handles which failure. Multiple VLANs, inconsistent roots and mismatched trunk allowances can complicate otherwise simple failover plans.

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