Spanning Tree Protocol

Learning objectives

• Explain the purpose and scope of Spanning Tree Protocol.
• Identify important protocols, roles, fields, and terminology.
• Apply the concept to a practical network scenario.
• Use IOS XE commands to verify operational behavior.

1. Core concepts

Layer 2 loops can cause broadcast storms, duplicate frames, and MAC-table instability because Ethernet frames do not have an IP TTL. STP creates a loop-free logical topology by placing selected redundant ports into a non-forwarding state.

2. How it works

Switches exchange BPDUs. The lowest Bridge ID wins the root bridge election. Each non-root switch selects a root port, its best path toward the root. Each segment has a designated port. RSTP improves convergence. PortFast is for edge ports and BPDU Guard protects edge ports from unexpected BPDUs.

Visual walkthrough

3. Worked example

If Switch A has the lowest Bridge ID, it becomes root. Each non-root switch selects its lowest-cost path toward A as root port. On a redundant segment, one port may become alternate and discard to prevent a loop.

IOS XE commands and verification

Syntax and feature availability can vary by device family and software release. Confirm platform support before applying configuration.

show spanning-tree
show spanning-tree vlan 10
show spanning-tree root
spanning-tree vlan 10 root primary
interface gi1/0/10
 spanning-tree portfast
 spanning-tree bpduguard enable

Common mistakes and troubleshooting

• Check physical and interface state before changing higher-layer settings.
• Verify both ends agree on addressing, VLAN, encapsulation, and protocol parameters.
• Use operational show commands; configured text alone does not prove a feature is working.
• Change one variable at a time and verify the result.

Knowledge check

Choose one answer per question. Submit to see why the correct choice is right and why every other option is wrong. These are original practice questions, not Cisco exam questions.