I am looking over some materials describing STP's port states (the original version from 802.1d) and they say that a port in Listening state can not modify the MAC address table (it doesn't learn new source MACs) and MAC addresses already in the table from which no frames are received on that port during this state, are removed (timed out). My question is: Given that a Listening state comes right after Blocking state where no source MACs are learned anyway, what would be a scenario that would result in a Listening port having MAC addresses learned in the table?
1 Answer
A switch can have learned MACs from other interfaces. So when a blocked port transitions to listening, there are already MAC addresses in the table and any MACs received on the listening port won't be added.
