What happens when 2 computers transmits at the same time to a 3rd one in a full duplex switch? Consider the following scenario: I have 2 PC's (PC1 and PC2) that wants to transmit at the same time to PC3 in a full duplex ethernet switch. Let's consider that all ports are in the same VLAN, so what happens internally in the switch? Who transmits first to PC3? I have read before that CSMA/CD was used, but only it was used in earlier Ethernet versions that operated in half-duplex, and each port of the switch was a collision domain and if 2 machines attempted to transmit at the same time, an algorithm was executed to give a random time to each computer to transmit and solve the collisions problem. However, in a full duplex switch I read that the possibility of a collision is eliminated so, if 2 PCs attempt to transmit at the same time, what happens internally in the switch? Does the switch execute an algorithm to choose who transmit first?

The switch will fully load the incoming frames of data, from the two sending systems, into its buffer(s). I'm not sure how it determines which frame would be first in the queue for subsequent forwarding; but it's probably based on initial receive time of the beginning of the frame. Then the switch works through the transmit buffer queue sending the frames out one-by-one onto the destination port/segment.

There's no issue with frames "running into each other." The real issue is can the ultimate port/segment accept the frames fast enough. (And, of course, can the switch process its buffer/queues fast enough.)