Spanning Tree Protocol (STP) — Interactive Learning Module

📖

What is Spanning Tree Protocol?

Understanding the purpose and history of STP

🎯 Learning Objectives After this module you will be able to: define STP and explain why it exists, describe the problems loops cause in Ethernet networks, identify STP variants (STP, RSTP, PVST+, MSTP), and relate STP to the OSI Data Link layer.

🌐 The Loop Problem

Ethernet networks use switches to connect devices. When switches are connected in a redundant topology (multiple paths between switches), a Layer 2 loop can form. This causes:

Broadcast storms — frames replicate endlessly
MAC table instability — switches learn incorrect ports
Multiple frame delivery — hosts receive duplicates
Network collapse — 100% bandwidth consumed instantly

🛡️ STP Solution

STP (IEEE 802.1D, 1990) solves the loop problem by logically blocking redundant links while maintaining physical redundancy. It:

Elects a single Root Bridge as topology reference
Calculates the best path to root from each switch
Places redundant links in Blocking state
Automatically reconverges when topology changes

OSI Layer 2 — Data Link

STP operates at the Data Link Layer (Layer 2) of the OSI model. It uses BPDU (Bridge Protocol Data Unit) frames with a destination MAC of 01:80:C2:00:00:00 (STP multicast) to communicate between switches.

📜 STP Variants Timeline

Standard	Full Name	Year	Convergence	Key Feature
802.1D	Original STP	1990	30–50 sec	Loop prevention baseline
PVST+	Per-VLAN STP Plus	Cisco	30–50 sec	Instance per VLAN (Cisco)
802.1w	Rapid STP (RSTP)	2001	1–6 sec	New port roles, rapid convergence
802.1s	Multiple STP (MSTP)	2002	1–6 sec	Maps multiple VLANs to instances
Rapid PVST+	Rapid Per-VLAN STP	Cisco	1–6 sec	RSTP + per-VLAN (Cisco default)

💡 Industry Note Modern Cisco switches run Rapid PVST+ by default. While original STP is less common today, its concepts are foundational for all variants and are tested on CompTIA Network+, CCNA, and CISSP examinations.

🧠

Core STP Concepts

Key terminology and operational principles

👑 Bridge ID (BID)

Every switch has a unique Bridge ID consisting of:

Priority: 2 bytes (default 32768)
Extended System ID: VLAN number (optional)
MAC Address: 6 bytes (tie-breaker)

        BID = Priority (32768) + MAC
e.g., 32768:00:1A:2B:3C:4D:5E
      

🌳 Root Bridge

The switch with the lowest BID becomes the Root Bridge — the reference point for the entire spanning tree topology. All paths are calculated relative to it.

Best practice: Manually assign the Root Bridge to a high-performance, centrally located core switch using a lower priority (4096 or 8192).

💰 Path Cost

STP calculates the best path to the root based on cumulative port costs. Lower cost = preferred path.

Speed	Cost (IEEE)
10 Mbps	100
100 Mbps	19
1 Gbps	4
10 Gbps	2

🔌 Port Roles

Root Port (RP) — Best path toward root bridge (one per non-root switch)
Designated Port (DP) — Best port on each segment toward root (forwards traffic)
Non-Designated / Blocked Port — Redundant port placed in Blocking state
RSTP adds: Alternate and Backup ports

⏱️ STP Timers

Hello Time — 2 sec: How often Root sends BPDUs
Forward Delay — 15 sec: Time in Listening and Learning states
Max Age — 20 sec: How long to store BPDU info before discarding

⚠️ Convergence Time: STP convergence can take up to 50 seconds (20 + 15 + 15) — a major drawback addressed by RSTP.

🔒 STP Enhancements & Security Features

PortFast Bypasses Listening/Learning on access ports (connected to end devices). Instantly moves to Forwarding.

BPDU Guard Disables a PortFast-enabled port if a BPDU is received — prevents unauthorized switch connections.

Root Guard Prevents external switches from becoming root bridge on a protected port.

Loop Guard Prevents Alternate/Root ports from becoming Designated if BPDUs stop arriving (prevents unidirectional link loops).

🗳️

Root Bridge Election Process

Step-by-step walk-through of how STP selects the Root Bridge

📋 Election Rules (in order of precedence) 1. Lowest Bridge Priority → 2. Lowest Bridge MAC Address (tie-breaker)

🔢 Bridge ID Calculator

Enter switch parameters to compute Bridge IDs and determine the Root Bridge winner.

📐 Root Port Selection (After Root Election)

On each non-root switch, the port with the lowest Root Path Cost becomes the Root Port. Ties are broken by:

Lowest Sender Bridge ID
Lowest Sender Port ID
Lowest Receiving Port ID

🏆 Designated Port Selection

For each network segment, the port with the lowest Root Path Cost on that segment becomes the Designated Port. If costs are equal, the switch with the lower BID wins. All Root Bridge ports are automatically Designated.

🔄

STP Port States

Understanding how ports transition through STP states

DISABLED

Admin off

→

BLOCKING

Max Age (20s)

→

LISTENING

Fwd Delay (15s)

→

LEARNING

Fwd Delay (15s)

→

FORWARDING

Active ✓

📊 Port State Comparison Table

State	Receives BPDUs	Sends BPDUs	Learns MACs	Forwards Data	Duration
Disabled	No	No	No	No	Until enabled
Blocking	Yes ✓	No	No	No	Max Age (20s)
Listening	Yes ✓	Yes ✓	No	No	Fwd Delay (15s)
Learning	Yes ✓	Yes ✓	Yes ✓	No	Fwd Delay (15s)
Forwarding	Yes ✓	Yes ✓	Yes ✓	Yes ✓	Normal operation

⚡ RSTP Port States (Simplified)

RSTP (802.1w) reduces states to just 3:

Discarding — Combines Disabled + Blocking + Listening
Learning — Same as STP Learning
Forwarding — Same as STP Forwarding

🔌 RSTP Port Roles

Root Port — Best path to root
Designated Port — Best port on segment
Alternate Port — Backup to Root Port
Backup Port — Backup Designated Port

RSTP achieves rapid convergence through proposal/agreement handshake mechanism.

🎮 Interactive Port State Explorer

Click any port state to learn what happens at that stage.

📦

Bridge Protocol Data Unit (BPDU)

Anatomy of the STP control message

📡 What is a BPDU? BPDUs are special Layer 2 frames exchanged by switches every Hello Time interval (2 seconds) to share STP topology information. They are sent to the STP multicast address 01:80:C2:00:00:00.

🎨 BPDU Frame Visual Map

Each colored block represents a field. Hover for details. Width is proportional to byte length.

📋 Configuration BPDU Fields

Field	Size	Value / Description
Protocol Identifier	2 bytes	Always 0x0000 (Spanning Tree Protocol)
Protocol Version	1 byte	0x00 = STP, 0x02 = RSTP, 0x03 = MSTP
BPDU Type	1 byte	0x00 = Config, 0x80 = TCN (Topology Change Notification)
Flags	1 byte	TC bit (bit 0), TCA bit (bit 7); RSTP adds bits 1–6
Root Bridge ID	8 bytes	Priority + Extended SysID + MAC of Root Bridge
Root Path Cost	4 bytes	Cumulative cost from sender to Root Bridge
Bridge ID	8 bytes	Sender's own Bridge ID
Port ID	2 bytes	Sender's port priority + port number
Message Age	2 bytes	Hops from Root Bridge (in 1/256 sec increments)
Max Age	2 bytes	Maximum age for stored BPDU info (default 20s)
Hello Time	2 bytes	Hello interval (default 2s)
Forward Delay	2 bytes	Listening/Learning duration (default 15s)

🚩 BPDU Types

Configuration BPDU — Sent by Root Bridge every Hello Time; propagated by all switches. Contains full topology info (35 bytes).
TCN BPDU (Topology Change Notification) — Sent toward Root when a topology change is detected (1 byte payload).
RST BPDU — Used by RSTP; combines config and TCN into single message type.

🏳️ Flags Byte Breakdown

          Bit 7
TCA
          Bit 6
Agr
          Bit 5
Fwd
          Bit 4
Lrn
          Bit 3
Role
          Bit 2
Role
          Bit 1
Prop
          Bit 0
TC
        
TC=Topology Change, TCA=TC Ack, Agr=Agreement, Fwd=Forwarding, Lrn=Learning, Prop=Proposal (RSTP bits 1-6)

🔧

STP BPDU Header Builder

Construct a complete BPDU frame field by field

🛠️ Instructions Fill in each field below using the valid values described. Click Build BPDU to generate the hexadecimal representation of your BPDU frame. This mirrors what a real switch would transmit.

🔧 BPDU Frame Constructor

IEEE 802.1D Configuration BPDU — 35 bytes total

Protocol ID 2 bytes

Protocol Version 1 byte

BPDU Type 1 byte

Flags 1 byte

Root Priority 2 bytes

Root Bridge MAC 6 bytes

Root Path Cost 4 bytes

Bridge Priority 2 bytes

Bridge MAC 6 bytes

Port ID 2 bytes

Message Age 2 bytes

Max Age 2 bytes

Hello Time 2 bytes

Forward Delay 2 bytes

🖥️

STP Network Simulator

Watch STP elect a root bridge and converge on a loop-free topology

// STP Simulation Log
Click "Next Step" or "Auto Run" to begin simulation...

📚

STP Glossary

Key terms and definitions for network administrators

❓

Knowledge Check Quiz

Test your understanding of Spanning Tree Protocol