After playing a little bit with a topology randomly created in packet tracer, I figured out that the switch (Switch0) stores a unknown device mac address on start out, wich I think that it belongs to a indirectly connected PT-Bridge interface. (See topology) I dont know if I am right. But the Switch0 also stores the mac address interface from the other switch (Switch1) on start out. I can check it by seeing this mac address in the Switch1 interface (Fa0/1). I have not chance to verify the bridge mac addesss to argue it, and I am not sure that bridge interfaces has got mac addresses assigned. Any comment that will clear this issue will be welcome! Thanks in advance!