3Com SuperStack 3 3250 Implementation Manual page 41

How STP Works 41
Figure 8 Port costs in a network
Bridge A has the lowest Bridge Identifier in the network, and has
therefore been selected as the Root Bridge.
Because Bridge A is the Root Bridge, it is also the Designated Bridge
for LAN segment 1. Port 1 on Bridge A is therefore selected as the
Designated Bridge Port for LAN Segment 1.
Port 1 of Bridges B, C, X and Y have been defined as Root Ports
because they are the nearest to the Root Bridge and therefore have
the most efficient path.
Bridges B and X offer the same Root Path Cost for LAN segment 2,
however, Bridge B has been selected as the Designated Bridge for the
segment because it has a lower Bridge Identifier. Port 2 on Bridge B is
therefore selected as the Designated Bridge Port for LAN Segment 2.
Bridge C has been selected as the Designated Bridge for LAN segment
3, because it offers the lowest Root Path Cost for LAN Segment 3: