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• Bridges exchange bridge
protocol data units (BPDUs). These have configuration messages consisting of:
– Root ID, bridge assumed by
sending bridge to be the root
– Transmitting bridge ID
– Cost of least cost path to the
root of which the transmitting bridge is aware
• When a bridge receives a
configuration message from a neighbor bridge, it compares this with what it
would transmit over that port. Note that it will add the cost to the received
message before comparison. It saves the best configuration
message received for each port.
If the saved configuration is better than what it would transmit it stops
transmitting BPDUs over that
• All bridges start by
transmitting on all ports:
– Root id is own id
– Transmitting id is own id
– Cost is 0
– (Port id of port)
– Which is a better message?
First compare root ID, lower is
better
If tie, next compare costs, lower
is better
If tie, next compare transmitting
ID, lower is better
If still tie, port id is tie breaker
Eventually only the root bridge is transmitting.
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