Editing Spanning Tree Protocol (section)

=== Path to the root bridge ===
The sequence of events to determine the best received BPDU (which is the best path to the root) is:

# Lowest root bridge ID (BID){{dash}}Determines the root bridge.
# Lowest cost to the root bridge{{dash}}Favors the upstream switch with the least cost to root
# Lowest sender bridge ID{{dash}}Serves as a tiebreaker if multiple upstream switches have equal cost to root
# Lowest sender port ID{{dash}}Serves as a tiebreaker if a switch has multiple (non-[[EtherChannel]]) links to a single upstream switch, where:
#*Bridge ID = priority (4 bits) + locally assigned system ID extension (12 bits) + ID [MAC address] (48 bits); the default bridge priority is 32,768, and
#*Port ID = priority (4 bits) + ID (Interface number) (12 bits); the default port priority is 128.