Hi this is straight off the Cisco site
This page will explain how incorrect information can be forwarded in a redundant switched network.
In a redundant switched network it is possible for switches to learn the wrong information. A switch can incorrectly learn that a MAC address is on one port, when it is actually on a different port. In this example the MAC address of Router Y is not in the MAC address table of either switch.
Host X sends a frame directed to Router Y. Switches A and B learn the MAC address of Host X on port 0.
The frame to Router Y is flooded on port 1 of both switches. Switches A and B receive this information on port 1 and incorrectly learn the MAC address of Host X on port 1. When Router Y sends a frame to Host X, Switch A and Switch B also receive the frame and will send it out port
- This is unnecessary, but the switches have incorrectly learned that Host X is on port 1.
In this example the unicast frame from Router Y to Host X will be caught in a loop.
What I cant get my head around is why/how the switches have learnt Host X is on port 1 not port 0? Is it because the switches learn the MAC from their port 1? That doesnt seem to make any difference.
TIA