Hi... I have a question that i can't find anywhere.
In spt if the root switch fails how the other switches know that it fails ? The other switches may know that topology changes and then they sent TC-BPDUs from their root port to the root switch? right ? But this message never reach it's destination. right? so.. how do they know? Thanks
With regular STP convergence, that's achieved by aging out a timer. Basically, the root is sending its information periodically. If the neighbor don't receive it in max-age (20 seconds), they consider that the root is gone.
For RSTP, detection is faster: if the link to the root bridge goes down (root port goes down) and if there is no alternate port, the root bridge is immediately considered gone. A new BPDU is sent to the neighbors, who immediately know that the root is gone, unless they have themselves an alternate port. This mechanism thus does not rely on any timer. If no link failure is detected but there is loss of connectivity to the root, then the usual STP timer (max-age) applies.