Hello Shivam
In the past, we used hubs in order to connect Ethernet hosts to the network. This meant that any frame sent by one host would be received by all other hosts. Hosts would then check the destination MAC address to see if the frame belongs to them. If not, it would be discarded. Hubs present another problem as well which takes place if two or more hosts send a frame to the network at the same time. The result is a collision. The group of hosts that will be reached by a frame sent by one of the hosts is called a collision domain. So a hub. and all the hosts connected to it would constitute a collision domain. CSMA/CD is the mechanism used to coordinate how hosts in a single collision domain communicate with each other.
For small networks, this worked well, but as networks got larger, hosts would be overwhelmed with frames that do not belong to them, and they would spend too many resources discarding such frames. The occurrences of collisions also increase with the increase in the number of hosts.
A switch unlike a hub, functions such that each of its ports makes one collision domain. This means that a switch port, and the host it connects to are the only two devices within that collision domain. It looks something like this:
Now, if the communication between a host and the switchport is configured as half duplex, then there is the possibility of having a collision, because both the host and the switchport may try to send data at the same time. CSMA/CD must operate in this case. However, if you configure the communication to function in full duplex, which allows data to flow in both directions at the same time, then you have eliminated the possibility of having a collision completely.
Half and full duplex on both switches and routers is configured using the duplex command in interface mode. You can configure it as either full, half, or auto. Full and half force the port to function in that mode, but auto will allow the port to negotiate with the host.
It is best practice to keep the configuration in auto for hosts and to configure it as full for connections from switch to switch, or switch to router to avoid any negotiation errors. Half duplex should never be used unless necessary for compatibility with older devices.
You will find much more information in the following lesson:
I hope this has been helpful!
Laz