First of all, we would like to welcome you again to a new article from our Cisco CCNA exam preparation series. Today we will study the Spanning-tree Protocol (STP).

Spanning-tree Protocol (STP) is a link-layer protocol. STP allows you to add redundant links to your network and maintain a loop-free topology. In your preparation for the CCNA exam you must learn how STP operates, how to configure and troubleshoot it.

STP is defined in the IEEE 802.1D standard. It is used to allow a network with redundant links to automatically enable/disable the failing links or just one of the active ones, leaving a single path for packets to travel across two network devices.

In bigger networks, having a redundant design is crucial. Redundant network designs allows service continuity in case of link failure. However, the most common you will encounter will be loops. Loops are created when multiple paths exist between two network devices. IP packets have a time to live (TTL), but Ethernet frames don’t. The result is that Ethernet frames will endlessly bounce between two switches if a loop is present in your network. The same goes for broadcast messages, but they’re effect is bigger, meaning that you have big chances to end with a Broadcast Storm. When you encounter a loop in your network, unicast frames are also affected. Loops make unicast frames duplicate, and the destination device will receive two identical packets. All these issues can be prevented if you use STP between your network switches.

STP uses the Spanning Tree Algorithm (STA) to make decisions which switch ports to put on blocking state to prevent loops in your network.

The STP network convergence consists in three steps: elect a root bridge, elect root ports and elect designated and non-designated ports.

When you enable STP, the first step in its convergence process is to elect a switch to act as a root bridge using the Spanning Tree Algorithm. To accomplish this, all network switches participation in STP exchange BPDU frames to determine which one has the lowest bridge ID (BID). The one with the lowest BID gets elected as a root bridge by STP. The root bridge is where all spanning-tree path cost calculations begin.

Understanding the STP root bridge election process is essential for your CCNA certification exam, so we will explain it to you in greater detail in part II of our CCNA Series covering STP.