Spanning Tree Protocol

Table of Contents

1. INTRODUCTION

The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and ensuing broadcast radiation.

2. ROLES

Spanning Tree defines 3 port roles. They are:

Root Port
Designated Port
Blocking (Alternative Port)

3. STATES

The following states show in sequential order the various states that a port transitions through once the port is opened.

Blocking – As the name suggests the port blocks all traffic and does not participate in frame forwarding / receiving. Due to this no MAC address are learnt. Though BPDU`s from neighbouring switches are excepted.
Listening – The port still cannot forward or receive frames but can receive and send BPDU`s.
Learning – Once the port is in the learning state the port starts learning MAC address.
Forwarding – When in the forwarding state the port sends/receives frames and BPDU`s in addition to populating its MAC address table.
Disabled – The port is administratively shutdown.

4. SPANNING TREE STEPS

Spanning Tree consists of the following steps:

Root Bridge election
Root Port election
Designated Port election
Alternative (Blocking) Port election

Root Bridge Election

The bridge with the lowest bridge ID is elected as the root bridge. The bridge ID comprises of the following:

Priority number
MAC address of bridge

Example: 32768.00:00:00:00:00:01 / [Priority number].[MAC Address]

During the Root Bridge election process the Priority number is compared first. If the Priority number is equal then the MAC address is then used.

Root Port Election

Each port that has the lowest path cost to the Root Bridge is assigned the role of the root port. The path cost is calculated by the accumulative cost of all paths to the Root Bridge.

Note :