Spanning Tree Protocol Explained: What Is STP in Networking?

What is the Spanning Tree Protocol (SPT)?

The Spanning Tree Protocol (STP) is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network (LAN). In essence the STP serves as a blueprint or compass to more efficiently navigate the network.

Picture a city with a complex network of bridges connecting its many islands. Without a blueprint to prevent the formation of closed loops, traffic would become entangled in an endless cycle, causing chaos and congestion throughout the city.

Similarly, STP maps out the best route for data packets to traverse the network, eliminating the potential for loops and ensuring efficient communication across network devices.

How Spanning Tree Protocol works

STP is built on bridge protocol data units (BPDUs), which are constantly sent back and forth between neighboring switches in the LAN and contain all STP data in their frames.

When transmitting BPDUs, a switch employs a distinct source MAC address associated with its originating port, targeting a multicast address characterized by a specific destination MAC. 

Any time a bridge is connected to the network or its topology changes, the bridge will receive a special BPDU requesting configuration.

STP operates on a hierarchical structure, with the establishment of a root bridge serving as the foundation. The root bridge is typically chosen automatically based on the lowest MAC address. This is often the oldest and slowest device, so you may want to select the root bridge manually.

Once determined, the root bridge becomes the reference point from which all other switches calculate their path costs.

Path costs are determined by the accumulated sum of individual link costs, with lower values representing more favorable routes.

Once the root bridge has been selected and the lowest path costs established, redundant paths are subsequently placed in a blocking state to prevent the formation of loops, while the remaining active paths facilitate the smooth flow of data traffic.

All bridges and switches that STP runs on are 802.1D-compliant.

5 STP port states

During the Spanning Tree Protocol’s operation, ports on network switches can transition between five distinct states, each serving a specific purpose in the quest for a loop-free topology: disabled, blocking, listening, learning, and forwarding.

1. Disabled: The port is administratively shut down and does not participate in STP.
2. Blocking: The port receives and processes BPDUs but does not forward data frames, effectively preventing the formation of loops.
3. Listening: The port is actively engaged in the election of the root bridge and designated ports, and will process incoming BPDUs, but still refrains from forwarding data frames.
4. Learning: While still not forwarding data frames, the port is now able to update its MAC address table based on the source addresses it receives.
5. Forwarding: In this final state, the port is fully operational and facilitates the flow of data frames and the processing of BPDUs.

4 STP modes

The Spanning Tree Protocol offers several modes of operation, catering to the diverse requirements of network managers.

• Common Spanning Tree (CST): A single instance of STP encompasses the entire network, regardless of the number of VLANs present. CST offers simplicity but lacks granular control and flexibility.
• Per-VLAN Spanning Tree (PVST): Unique to Cisco devices, PVST enables the creation of separate spanning trees for each VLAN. PVST provides a higher degree of control, but at the expense of increased resource consumption.
• Per-VLAN Spanning Tree Plus (PVST+): An enhancement of PVST, PVST+ allows for interoperability with non-Cisco devices implementing the IEEE 802.1Q standard.
• Multiple Spanning Tree (MST): A highly efficient mode that enables the grouping of multiple VLANs into a single Spanning Tree instance, reducing resource usage and management complexity.

3 STP timers

Three fundamental timers govern the operation of the Spanning Tree Protocol, ensuring timely and efficient convergence of the network.

• Hello Timer: The interval at which the root bridge transmits BPDUs to neighboring switches, typically set to 2 seconds. 
• Forward Delay: The duration a port spends in both the Listening and Learning states before transitioning to the Forwarding state, with a default value of 15 seconds.
• Max Age: The maximum time a switch retains a BPDU before considering it stale and discarding it, set to 20 seconds by default.

Is enabling STP worth it?

The decision to enable SPT depends on the specific needs and objectives of your enterprise network. STP is particularly useful for enterprise networks with redundant paths, where the risk of loops and broadcast storms is imminent. However, in smaller networks with minimal redundancy or in networks with well-defined Layer 3 boundaries, STP may not be as crucial.

Advantages of STP

The Spanning Tree Protocol offers several notable benefits to network managers:

• Loop prevention: STP’s primary function is to eliminate loops, ensuring a stable network topology and preventing broadcast storms.
• Redundancy: By selectively blocking and unblocking ports, STP enables the efficient use of redundant paths, enhancing the network’s fault tolerance.
• Scalability: STP can accommodate the addition of new switches or VLANs, dynamically adjusting the network topology as needed.
• Simplifies bridge logic: STP simplifies bridging logic by establishing a root bridge that sees all traffic in the network and ensures efficient data forwarding.
• Backups: It also provides backups that become active when the main connection experiences technical hiccups.

Disadvantages of STP

Despite its advantages, STP has certain limitations and drawbacks:

• Convergence time: STP’s convergence can be relatively slow, especially in large networks, potentially leading to temporary disruptions in data traffic.
• Inefficient use of links: Blocked ports result in wasted bandwidth, as they remain inactive until a topology change occurs.
• Complexity: The configuration and management of STP can be intricate, particularly in networks with multiple VLANs and spanning tree instances.

What is Rapid Spanning-Tree Protocol (RSTP)?

The Rapid Spanning Tree Protocol (RSTP), defined by the IEEE 802.1w standard, is an evolution of the classic STP. 

RSTP aims to address some of STP’s shortcomings by providing faster convergence times and enhanced efficiency. By introducing features such as alternate and backup ports, RSTP can rapidly respond to changes in the network topology, reducing convergence time and minimizing disruptions.

Are there alternatives to STP?

While STP and its variants remain popular choices for loop prevention, there are alternative technologies that can achieve similar objectives, such as Shortest Path Bridging (SPB) and Transparent Interconnection of Lots of Links (TRILL).

Shortest Path Bridging (SPB)

Based on the IEEE 802.1aq standard, SPB combines the benefits of OSI Layer 2 and Layer 3 protocols, offering a simplified and scalable solution for loop prevention and network management.

One of the key features of SPB is its utilization of Dijkstra’s algorithm, a graph theory-based algorithm designed to find the shortest path between nodes in a weighted graph. By implementing Dijkstra’s algorithm, SPB calculates the optimal routes between switches, ensuring efficient data traffic flow while simultaneously eliminating the risk of loops.

Moreover, SPB enhances network flexibility and resilience by supporting multiple equal-cost paths, thus providing improved load balancing and fault tolerance capabilities.

Transparent Interconnection of Lots of Links (TRILL)

Based on the IETF RFC 6326 standard, TRILL employs shortest path routing protocols at Layer 2 of the OSI model and supports multihopping environments. It can work with any network topology, using links that would otherwise have been blocked, and can be used at the same time as STP. In fact, it was designed by the same person, Radia Perlman, as a successor to STP.

The main benefit of TRILL is that it frees up capacity on your network which can’t be used (to prevent routing loops) if you use STP, allowing your Ethernet frames to take the shortest path to their destination. This in turn means more efficient utilization of network infrastructure and a decreased cost-to-benefit ratio.

These benefits are particularly important in data centers running cloud computing infrastructure. TRILL is also considered more stable than STP because it provides faster recovery time in the event of hardware failure.

Bottom line: Using STP in enterprise networks

The Spanning Tree Protocol, with its ability to ensure a loop-free network topology, remains a critical tool in the arsenal of network managers.

While STP is not without its drawbacks, the introduction of Rapid Spanning Tree Protocol and alternative technologies offers additional options for achieving network stability and optimization.

A thorough understanding of STP and its variants will enable network managers to harness their networks’ full potential and make informed decisions that cater to their network’s unique requirements.

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