This is a quick note to help me understand STP with trunks.

Suppose you have two switches, switchA and switchB. They both have two trunks between each other. Say I connect nodeA and nodeB to switchA and switchB. I then send a packet from nodeA to nodeB on switchA. SwitchA will not know where to send that frame as it does not have the MAC address so it uses ARP. SwitchA broadcasts the MAC address to every port except on which the frame arrived on. The MAC goes out through one of the trunk ports towards switchB. SwitchB looks at the frame and search for a node with that MAC address. It doesn't see one thus it sends out the MAC address to all the ports other than the one it received on. That frame goes out towards switchA which then causes a loop.

Spanning Tree Protocol avoids that by closing off one of the trunks so the frame with the MAC will not be able to go back towards switchA from switchB.


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