How spanning tree protocol (STP) selects a designated port

Spanning tree protocol (STP)

The spanning tree protocol (STP) is a communication protocol used to create a loop-free topologyThe arrangement of elements in a computer network.. STP is present mainly in layer-2It is a device that operates according to the Layer 2 data communications protocol. bridges and switches. It also serves as a backup connection for the network system if the primary link fails.

Each bridge's root port is a branch of the spanning tree. The ports chosen by the spanning tree are the best options for reaching the root bridge, also known as the destination port. That is, each switch has just one root port. If any other switch lacks a root port, it chooses one specified port and two non-designated ports. The specified port will be regarded as forwarding, while the others will be considered blocking.

STP allows a single node's port while disabling all others. It signifies that there is just one active transmission line between the two nodes. The STP-selected port is the root port and transfers data to the root bridge. The root bridge gets all data from the other bridges.

The port cost, priority, and switch ID determine the path with the lowest price to the respective destination.

Steps

The steps for selecting the targeted port are as follows:

1. Choose the switch with the cheapest route cost: We must choose the switch with the lowest route cost.

2. Choose the switch's allocated port depending on the lowest price: The destination may reach through two ports. We must choose the lowest-cost port as the designated port and another as the non-designated port.

3. Specify the non-designated port: The other switches' root port is the NDP (Non-designated port), which is in a non-forwarding or blocking condition. It is to avoid data loops during transmission.

Explanation

One of the switches, switch3, and switch1, must be selected as the designated port, and the other as the non-designated port. Switch3 is chosen as the specified port in the forwarding state since it has the lowest cost (10<18). Switch1 is selected as the non-designated port that is blocked.