What is a Root Port
If a switch is selected as a Root Switch (Root Bridge), then all other switches in that network must have to select a single port in each of the switches. This selection of the single port in every switch is done to reach the Root Switch.
The switch that has a single selected port which has the minimum path coast will be then called the Root Port. One important thing is that the Root Switch does not contain the Root Port. This is because of the reason, that the Root Bridge is occurred at the root and therefore, there is no need to reach at the root by the root port.
If a switch containing more than one ports to reach at the Root Switch, it needs to select one single port for the transmission. This selection of the single port can be done with the help of the Spanning Tree Protocol Algorithm. The selection of the port is selected based on the criteria, the path, from the switch to the root switch, having the minimal path coast.
All ports of the root bridge will be designated port. No ports will be root port there. If a switch has a root port it implies that it is the way towards the root bridge. Where the traffic forwarding is done by the designated port.
Consider that the switch 4 wants to transmit packets to root switch(switch 1). If there are more than one paths occurred to reach the destination, then it has to select an optimal path from the source to the destination. The optimal path indicates the path which has the minimal cost. Here, in this case, the switch 4 has two paths to transmit the packet to the destination, switch 1, either through switch 3 or through switch 2.
And It has two ports: port start from switch 4 to switch 3 and the port which start from the switch 4 to switch 2. Here the switch 4 has to select the optimal path. Here, in this case, the optimal path is from switch 4 to switch 3. Therefore the port where this path connected will be the root port of that particular switch(switch 4).
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When the packet reaches the switch 3, it then forwards to all the neighboring switches except the switch which it had sent. So there is no ambiguity in transfer, it then forwards to the switch 1. And here in switch 3, the port which connects the switch 1 is the root port.