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Understanding Spanning Tree Protocol

May 23, 2025

Jeremy's IT Lab: CCNA Spanning Tree Part 2

Introduction

  • Continuation of spanning tree studies.
  • Focus on the operation of spanning tree, spanning tree states, timers, BPDU, optional features (spanning tree toolkit), and configurations.

Spanning Tree States

  • Stable States:
    • Blocking: Non-designated ports, prevents loops, does not forward traffic, receives BPDUs.
    • Forwarding: Sends and receives traffic and BPDUs, learns MAC addresses.
  • Transitional States:
    • Listening: 15 seconds by default (Forward delay timer), only sends/receives BPDUs, no traffic or MAC learning.
    • Learning: 15 seconds by default, like Listening but learns MAC addresses.
  • Other States:
    • Disabled: Administratively shut down.

Spanning Tree Timers

  • Hello Timer: Determines BPDU send intervals, default every 2 seconds.
  • Forward Delay Timer: Length of Listening and Learning states, default 15 seconds each.
  • Max Age Timer: Time an interface waits without receiving BPDUs before reevaluating, default 20 seconds.

Bridge Protocol Data Unit (BPDU)

  • Fields:
    • Protocol Identifier: 0000
    • Protocol Version Identifier
    • BPDU Type
    • Flags
    • Root Identifier: bridge priority, MAC address, etc.
    • Root Path Cost
    • Port Identifier
    • Timers: Message age, Max age, Hello, Forward Delay.

Optional Spanning Tree Features

  • PortFast: Bypasses Listening and Learning states for ports connected to end hosts, risks loops if connected to another switch.
  • BPDU Guard: Shuts down ports receiving BPDUs to prevent loops.
  • Root Guard: Prevents a port from becoming a root port if superior BPDU is received.
  • Loop Guard: Prevents interface from causing loops if it stops receiving BPDUs.

Spanning Tree Configuration

  • Mode Configuration:
    • MST, PVST, Rapid-PVST.
    • Rapid-PVST is default on modern Cisco switches.
  • Root Bridge Configuration:
    • Use SPANNING-TREE VLAN <number> ROOT PRIMARY/SECONDARY.
    • Helps with load balancing across VLANs.
  • Port Settings:
    • Cost and Port Priority can be set per VLAN.

Practice and Quiz

  • Suggested using Packet Tracer for practical labs.
  • Key quiz questions reviewed, focusing on PortFast, BPDU Guard, and spanning tree timers.
  • Recommended Boson ExSim for practice exams.

Conclusion

  • Encourage studying and practicing with provided materials (flashcards, lab videos).
  • Acknowledgement of supporters and call to action (subscribe, share, etc.).

Additional Resources

  • Review flashcard deck available.
  • Packet tracer lab for hands-on practice.