> Scenario A: Switch 1 has ports 1+2 trunked together via LACP. They are plugged into Switch 2 ports 1+2.
Both links will be used, but a single flow will only use one link. There is a hashing algorithm that determines which flow uses which link. If one of the links goes down, then all the traffic will be switched to the other link. This happens very rapidly.
> Scenario B: Switch 1 has ports 1+2 trunked together without LACP. They are plugged into Switch 2 Ports 1+2.
They will not actually be trunked together. STP will block one link because it creates a single, loop-free path to the root bridge. When the active link goes down, STP will switch over to the redundant link, but this happens fairly slowly; a few seconds for RSTP, and up to 50 seconds for standard STP.