Spanning Tree Protocol Concepts

 

Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on bridges and switches. The specification for STP is IEEE 802.1D. The main purpose of STP is to ensure that you do not create loops when you have redundant paths in your network. Loops are deadly to a network.

In this figure we can see, we use two copper cross over cable. Because if one cable down then another cable can transfer data. When we use two cable there can be 3 problem.

Those are

    Broadcast Strom

    Multiple Frame Copies

    CAM table Instabilities

To avoid those problem we use STP and it build in every switch.

How STP works

1^st step: Find Root bridge switch

Which switch has the lowest MAC address is the root bridge switch. In figure we can see, switch0 has 0009.7CB9.000 is the lowest MAC address of two switch. So switch0 is the root bridge.

 

2^nd step: Find Root port

Cost to reach root switch. In figure there are two path to reach root switch. So the lowest path will be the Root Port. If any time two path has same cost value then we have find lowest interface number will be the Root Port.

 

3^rd step: Designated Port

Cost to reach Root Switch. In figure there are two path to reach root switch. So the lowest path will be the Designated Port. If any time two path has same cost value then we have find lowest interface number will be the designated port.