How would a switch know that it should forward a frame through a different interface in a STP topology?

Question

Imagine the following STP topology

If PC0 was to ping PC1, all of the switches would create entries in their MAC table according to the source MAC address of the PCs associated with the corresponding interfaces.

Since ping consists of an ICMP Echo Request and Reply, the switches would have entries for both computers in their MAC address table and traffic would flow using this path from PC0 to PC1 and vice-versa.

However, what if S1's Fa0/2 interface failed? Wouldn't S2 still try to forward traffic towards S3 because of its MAC address table entry?

Answer 1

A few things you might not be aware of:

• spanning tree convergence takes place before any traffic is forwarded through the network
• the MAC/source address table in each switch is learned with the spanning tree topology already applied
• switching/bridging works the exact same way, regardless of whether STP is active or not - STP only controls the port states and blocks ports with redundant loops
• if a forwarding port's link state changes, the switch/bridge recalculates its port states and issues a topology change notification (TCN) to its neighbors, causing the other switches to reconsider their port states as well - the spanning tree reconverges
• when a switch port's state changes to down or blocked, its MAC entries are removed from the SAT and the according MAC addresses are relearned.