Note: A sends to B, table is switches are not filled: flooding of packages, switches will learn the network. B sends to A: switches know which interface can be used to reach a → only coorect path.

Note2: Switches have to connect LANs with Root switch. Switches B2 and B3 are not connecting any LANs with Root, therefore than are not active at all → Only LANs can be Leaves of the spanning tree.