In order for Spanning-Tree to operate, Bridge Protocol Data Units (BPDU) are used to communicate between all switches. Switches must learn of other switches in the network. This is accomplished by each switch sending a BPDU frame out a port and using a unique MAC address of that particular port as the "source address". Given that the switch is unaware of other switches in the network, BPDU's are sent with a "destination address" of the STP multicast address 01-80-c2-00-00-00.
The 2 Types of BPDUs are:
- Configuration BPDU
- Topology Change Notification (TCN) BPDU
The 2 Goals of BPDU's are:
- Elects reference points as the foundation for a stable spanning-tree topology.
- Helps spanning-tree communicate topological changes to deal with loops and faults in the switched network.
Configuration BPDU are used to compute spanning-tree. It is how the switches in the network agree on which switch will be the root bridge. The BPDU's are pass amongst each switch every 2 seconds. Once the root bridge is determined spanning-tree uses BPDU's to communicate topological changes such as links going down and coming back up.
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