Spanning Tree Costs
Link Speed Old Cost New Cost
10 Mbps 100 100
100 Mbps 10 19
1 Gbps 1 4
10 Gbps 1 2
The STP Election
Spanning Tree builds paths out from a starting point, the “root” of the tree.
The first step in selecting paths is to identify this root device. Then, each
device selects its best path back to the root, according to the criteria laid out
in the previous sections (lowest root BID, lowest cost, lowest advertising
BID, lowest port).