5.4 STP in the Campus Network
5.4.2
Determining the best loop-free path to the root bridge

After the root bridge has been elected, all switches determine the best loop-free path to the root. STP uses several different costs in determining the best path to the root bridge as follows:

  • Path cost

  • Bridge ID

  • Port priority

As the BPDU leaves a port, the BPDU applies a port cost. The sum of all of the port costs is the path cost. Spanning Tree looks first at the path cost to decide which ports should forward and which ports should block. If the path cost is equal for multiple ports, then Spanning Tree looks at the bridge ID. If the bridge IDs are equal, then STP compares the port IDs. The port with the lower port ID has priority, making that port the forwarding port. Cisco allows you to modify the first byte of the port ID by changing the port priority.