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Loops are bad for networks. Data that cannot find its way to the destination ends up getting bounced all over the network and causing congestion as it is passed around and around in circles. The spanning tree algorithm was developed for networks needing more than one bridge to connect their networks and to do so in a fashion that prevents loops. Spanning tree's purpose is to automate the process of calculating loop free paths through the physical networks to each LAN. This reduces the administrative overhead of having the network administrator manually configure all the network devices to perform loop free forwarding.

Spanning Tree Algorithm

The Spanning Tree Algorithm is designed to provide a loop free path from any given switch to the root bridge. For any two LANs there should be one and only one loop-free path between them.

Types of Spanning Tree

• Open Standards
  • 802.1D - Spanning Tree (STP)
  • 802.1s - Multiple Spanning Tree (MSTP)
  • 802.1w - Rapid Reconfiguration Spanning Tree (RSTP)
• Proprietary
  • Cisco PVST
  • Cisco PVST+

Basic Spanning Tree (802.1D) Election & Setup Process

1. Elect one bridge as the root bridge (this is the bridge with the lowest ID). This is the only bridge on ALL the connected LANs that will perform the root bridge function.
2. Once the root bridge is elected, the remaining bridges calculate the shortest path to the Root Bridge.
3. One bridge on each LAN is then elected as the DESIGNATED BRIDGE for that LAN. The elected bridge should have the shortest path to the root bridge. It is the designated bridge's job to forward datagrams to the root bridge.
4. Each bridge should will then choose a port that is the best path to reach the root bridge.
5. Each bridge should select ports that will participate in the spanning tree. Participating ports will be those ports on which 'self' has been elected as the designated bridge.
6. All ports begin forwarding data to the correct lans via the shortest, loop free path to the destination LAN.

Spanning Tree States

• Blocking
• Listening
• Learning
• Forwarding
• Disabled (not really a state, the port is disabled and administratively down)

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