NetSS

Four-Step STP Decision Sequence
When creating a loop-free logical topology, Spanning Tree always uses the same
four-step decision sequence:
Step 1.
Lowest Root BID
Step 2.
Lowest Path Cost to Root Bridge
Step 3.
Lowest Sender BID
Step 4.
Lowest Port ID
Bridges pass Spanning Tree information between themselves using special frames
known as bridge protocol data units (BPDUs). A bridge uses this four-step decision
sequence to save a copy of the best BPDU seen on every port. When making this
evaluation, it considers all of the BPDUs received on the port as well as the BPDU
that would be sent on that port. As every BPDU arrives, it is checked against this
four-step sequence to see if it is more attractive (that is, lower in value) than the
existing BPDU saved for that port. If the new BPDU (or the locally generated BPDU)
is more attractive, the old value is replaced.