Spanning Tree Protocol (STP)

Overview

Spanning Tree Protocol (STP) is a network protocol that prevents loops in Ethernet networks by disabling redundant paths. It ensures a single path between any two network switches, avoiding broadcast storms and network congestion.

Key Concepts

STP Purpose

Loop Prevention: Disables redundant links to prevent switching loops
Redundancy: Maintains backup paths that activate if the primary path fails
Network Stability: Ensures consistent network topology

How STP Works

Bridge Election: Selects a root bridge using Bridge ID (priority + MAC address)
Path Selection: Determines lowest-cost paths to the root bridge
Port Roles Assignment:
- Root Port: Shortest path to root bridge (per switch)
- Designated Port: Forwards traffic toward root bridge (per segment)
- Blocked Port: Unused ports that prevent loops

BPDU (Bridge Protocol Data Unit)

Control frames exchanged between switches
Contains information about the root bridge and path costs
Sent every 2 seconds by default

STP Variants

802.1D (Original STP)

Standard Spanning Tree Protocol
Convergence time: 30-50 seconds
Limited scalability

Rapid STP (RSTP - 802.1w)

Improved convergence: 1-2 seconds
All ports participate in topology change detection
Active on most modern networks

Multiple STP (MSTP - 802.1s)

Groups VLANs into instances
Reduces overhead for large networks
Better scalability

Bridge Priority

Bridge ID Components

Priority: 0-65,535 (default: 32,768)
MAC Address: Unique identifier per switch

Calculation

Priority = Bridge Priority Value + VLAN ID
(Lower value = higher priority)

Port States

State	Duration	Purpose
Disabled	N/A	Port administratively down
Blocking	20 sec	Receives BPDUs, no forwarding
Listening	15 sec	Prepares to forward
Learning	15 sec	Learns MAC addresses
Forwarding	Active	Forwards frames and BPDUs

Cost Calculation

Path Cost (802.1D Default)

Bandwidth	Cost
10 Mbps	100
100 Mbps	19
1 Gbps	4
10 Gbps	2

Lower cost = preferred path

Configuration Examples

Set Bridge Priority

Switch(config)# spanning-tree vlan 1 priority 4096

Set Port Cost

Switch(config-if)# spanning-tree cost 10

View STP Status

Switch# show spanning-tree
Switch# show spanning-tree detail
Switch# show spanning-tree vlan 1

Common Issues & Solutions

Slow Convergence

Problem: Network unresponsive after topology change
Solution: Implement RSTP or tune STP timers

Unnecessary Blocking

Problem: Some ports remain blocked unnecessarily
Solution: Adjust bridge priorities and port costs

Broadcast Storms

Problem: Excessive network traffic
Solution: Verify STP is running and converged

Best Practices

✓ Use RSTP (802.1w) in modern networks ✓ Set root bridge explicitly (priority 0 or 4096) ✓ Configure secondary root bridge for redundancy ✓ Monitor STP convergence regularly ✓ Document network topology ✓ Test failover scenarios ✓ Tune timers only when necessary

CCNA Exam Tips

Know STP port roles and states
Understand Bridge ID calculation
Be able to identify root bridge election
Know default STP timers (2, 15, 15, 20 seconds)
Understand RSTP improvements over 802.1D
Recognize topology change scenarios