How Spanning Tree (STP) Works? | How to Choose Root Port, Designated Port, Blocking Port?

STP (Spanning Tree Protocol) was developed to prevent loops in the Layer 2 layer.

How Spanning Tree (STP) Works

Working Logic of STP Protocol

Redundancy increases the availability of network topology by protecting the network from a single point of failures, such as a failed network cable or switch. When physical paths are added to network design, Loops are generated and frames are replicated. This creates serious problems for switched networks. Therefore, the Spanning Tree Protocol (STP) was developed to resolve these issues.

The STP ensures that there is only one logical route between all network destinations by deliberately blocking unnecessary routes that may cause a loop. A port is considered blocked when user data is not allowed to enter or exit this port. This does not include the bridge protocol data unit (BPDU) frames used by the STP to avoid loops.

Blocking unnecessary routes is very important to avoid loops in the network. Physical paths for redundancy still exist, but they are disabled to prevent loops from occurring. If a route is required to compensate for the failure of a network cable or a switch, the STP recalculates the routes and opens the necessary ports to allow activation of the spare route.

The use of the term “Spanning Tree Protocol” and the abbreviation STP can be misleading. Most experts often use these expansions to refer to various implementations of the expansion tree, such as the Fast Expansion Tree Protocol (RSTP) and the Multiple Expansion Tree Protocol (MSTP).

In order to accurately describe the concepts of Spanning Tree, it is important to refer to the application or context in a specific standard. IEEE-802-1D-2004, the latest IEEE document in the Spanning Tree tree, used the phrase “STP was replaced by the Fast Spanning Tree Protocol (RSTP).” As we have seen, IEEE uses “STP için to refer to the original implementation of Spanning Tree, and“ RSTP için to identify the version of the STP specified in IEEE-802.1D-2004. Then, when the original STP protocol is analyzed, the expression “original 802.1D expansion tree kullanılır is used to avoid confusion.

On Cisco Switches connected by a crossover cable, STP is automatically activated and prevents possible cycles. Port States are executed as soon as the STP protocol is started. The interface of one of the interconnected switches is set to Blocking mode and the other interface is set to Designated Port.

Now you can watch the Packet Tracer tutorial video below to get a better understanding of STP’s working logic and subscribe to our YouTube channel to support us!

   Final Word


After watching the video above, you should understand the basic logic of the Spanning Tree protocol. Although the STP protocol has a somewhat complicated configuration, it is very simple. Thanks for following us!

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