Spanning Tree ProtocolImplementing Cisco Networks

Outline

IntroductionWhy Use STP?Spanning Tree TermsSpanning Tree Operation

(STA)STP Port StatesSTP TimersSTP Versions

CCNA Project Report 2

Introduction

Layer 2 Bridge-to-Bridge ProtocolLoop Avoidance MechanismDeveloped by Radia PerlmanDefined in IEEE 802.1d standardEnabled by default on Cisco

Catalyst switchesUses the Spanning Tree AlgorithmCreates Loop-free Logical

TopologyCCNA Project Report 3

Why Use STP?

The Need for STPIn a layer 2 switched network, while multiple links betweendevices provide protection against single points of failure,they also introduce physical loops in the network.Redundant links can cause a lot of problems, including:

• Broadcast storms• Multiple copies of Ethernet frame• Thrashing of the MAC table

STP prevents physical networking loops from occurringby identifying the redundant links in the network, and blockingsome of them to create a loop-free logical topology.

CCNA Project Report 4

Spanning Tree Terms

Root Bridge – the focal point of the network that influences all STP decisions

Root Port – the port (on a nonroot bridge) used to reach the root bridge

Designated Port – the port with the least advertised path cost to the root bridge

CCNA Project Report 5

Spanning Tree Terms (contd.)Bridge ID – Priority + MAC

Address, used to identify each switch in the network, default priority = 32,768

BPDU – Bridge Protocol Data Unit, used by switches to exchange STP information among themselves

Path/Port Cost – determined by the bandwidth of a linkCCNA Project Report 6

Spanning Tree Operation (STA)

Step 1: Electing a Root Bridge• Bridge Priority, Bridge ID, Root

Bridge

Step 2: Electing Root Ports• Path/Port Cost, Root Path Cost, Root

Port

Step 3: Electing Designated Ports

• Path/Port Cost, Root Path CostCCNA Project Report 8

STP Port States

Disabled – Administratively downBlocking – Receives BPDUs onlyListening – Building active

topologyLearning – Building bridging tableForwarding – Building MAC table,

sending/receiving user data

CCNA Project Report 9

STP Timers

Hello Time – how often BPDUs are sent (default = 2 seconds)

Max Age – maximum time to retain the received BPDU information (default = 20 seconds)

Forward Delay – time to wait before transitioning from listening to learning state, and from learning to forwarding state (default = 15 seconds)

CCNA Project Report 10

STP Versions

Spanning Tree Protocol (IEEE 802.1d)

Per VLAN Spanning Tree (PVST+)Rapid Spanning Tree Protocol, or

RSTP (IEEE 802.1w)

CCNA Project Report 11

Thank You

Presented by:Premdeep S. GillRoll# LU-0453-10

Batch# 100223

CCNA Project Report 12