# Spanning Tree Protocol | What is STP

Spanning Tree Protocol (STP) is a network protocol developed to prevent network loops in a local area network (LAN). A loop can occur when there are multiple paths between two network switches or routers that send packets back and forth endlessly, causing network congestion and eventually network outages. STP works by selectively blocking some of these paths, creating a seamless topology, and ensuring that all network traffic is routed along a single path,  eliminating network congestion and reducing the risk of network outages.

STP works by creating a tree-like structure that spans all the switches in the network. This structure ensures that there is only one active path between two switches at any given time and is achieved using Bridge Protocol Data Units (BPDUs). BPDUs are messages that connect switches to determine the network topology and identify the root switch. The root switch is the switch with the lowest bridge ID, which is a combination of the switch's MAC address and a priority value.

Once the root switch is identified, STP chooses a single path from the root switch to every switch in the network based on a rule that prioritizes the path with the lowest cost. The cost of the path is determined by the link speed and the number of hops needed to reach the root switch. The path with the lowest cost is chosen as the active path, and all other paths are set to block, effectively blocking them.

When a link fails, STP reconfigures the network topology by choosing an alternate path to the root switch. The time it takes to reconfigure the network depends on the STP convergence time, which is the time it takes for switches to transition from blocking to forwarding. This time can be several seconds, during which the performance of the network can be reduced or interrupted. Several improvements have been made to the STP protocol, including Rapid Spanning Tree Protocol (RSTP) and Multiple Spanning Tree Protocol (MSTP), to mitigate the impact of convergence time.

RSTP is an improvement over STP that reduces convergence time by quickly moving the network from blocking mode to forwarding mode. This is achieved by exchanging BPDUs more frequently and removing the need for listen and learn mode. RSTP also supports a backup path when the active path fails, reducing downtime and ensuring network availability.

MSTP is a further improvement of the STP protocol that allows the creation of multiple logical networks within a single physical network. This is achieved by creating multiple instances of the STP protocol, each with its own root switch and path selection algorithm. Each logical network is assigned a unique VLAN ID and switches are configured to participate in one or more VLANs. MSTP improves network efficiency by allowing traffic to be routed along multiple paths, which increases network bandwidth and improves network flexibility.

In summary, Spanning Tree Protocol is a critical network protocol that prevents network loops and ensures network availability. To achieve this, it creates a loop-free topology and chooses the optimal path to the root switch according to the rules. The protocol has been improved over time with the introduction of RSTP and MSTP to reduce convergence time and improve network efficiency. Understanding STP is critical for network administrators because it is the foundation of many other network protocols and technologies.