Spanning-Tree Root Bridge

Spanning Tree Root Bridge

As soon as the switch powers up it first assumes that it is the Root Bridge. The Spanning Tree election process begins quickly given that BPDU's are sent out every 2 seconds.  Every switch begins sending out BPDU's with a Root Bridge ID equal to it's own Bridge ID and a Sender Bridge ID that is its own Bridge ID. The purpose of the sender ID is to inform the other switches who is actually the sender of the BPDU message.

Each BPDU will be checked to see if it has a lower Root Bridge ID. If the BPDU is lower than the current switches BPDU, the current switch will then replace its own Root Bridge ID with the newly announced Root Bridge ID.  If the Root Bridge ID is not lower then it will simply just forward the BPDU without making any changes.

As soon as the root bridge is determined, then configuration BPDU's will only be sent by the Root Bridge. The other switches must forward or relay the BPDU's while adding their own Sender Bridge ID to the message.