BRIDGES
1.6.2Spanning Tree Algorithm
The Spanning Tree Algorithm (STA) is used by bridges to detect data loops (duplicate data paths). The bridges will then automatically break the loop and use the now open path as a backup in case the primary path fails.
When a bridge is powered up, it goes through a series of self tests to check its internal operation. During this time the bridge is in a standby, or blocking, condition and does not forward traffic. Also during this standby period, the bridge sends out special bridge management packets called configuration Bridge Protocol Data Units (BPDU). Bridges use the BPDUs as a way of communicating with each other. The purpose of the configuration BDPU is to notify other bridges on all of the connected networks of the current topology.
After the bridge has informed the network of its presence, the bridge enters a second standby state, called listening. During listening, the bridge monitors the network for the BPDUs of other bridges. Having received packets from the networks, the bridge enters the learning state, continuing to block traffic as it examines the information it receives.
Based on bridge priorities and MAC addresses, the interconnected bridges will set bridge ports to either forwarding or standby conditions, allowing a single access path to all parts of the network. The bridge or bridges involved in this primary data path will then remain in the forwarding state, and the bridges with lower priority involved in the backup path(s) will remain in a standby condition. Any redundant paths (those placed in standby) will be automatically used as need is detected by the operation of the Spanning Tree Algorithm.
The other type of BPDU is the topology change BPDU. This BPDU is made up of four bytes and notifies the other bridges that a change has taken place. Upon receipt of the topology change BPDU, the bridges
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