Ditch the Confusion! Understanding the Latest Topology Changes in Spanning Tree Protocol (STP)


The complexity of modern networks demands high adaptability and efficiency, making the understanding of Spanning Tree Protocol (STP) a crucial skill for any network professional.

STP enables Ethernet switches to create loop-free topologies that prevent broadcast storms and ensure optimal network performance. One key aspect of STP is handling topology changes - adjustments made in response to link status or switch failures.

This blog post will explore the different types of topology changes in STP, their impact on your network's communication, and methods for effectively monitoring and troubleshooting these changes.

By gaining a firm grasp on this vital aspect of networking protocols, you'll be better equipped to maintain seamless connectivity across your organization's infrastructure.

Understanding Topology Changes in STP

Topology changes in STP refer to the modifications made to the network topology due to link status changes caused by factors such as switch or link failures, which can cause communication issues among switches.

Definition and Causes of Changes

Topology changes in Spanning Tree Protocol (STP) are crucial events that affect the overall stability and performance of a network. By definition, a topology change occurs when there's an alteration in the physical or logical structure of the network, such as when a new switch is added, a link between switches fails, or an existing connection is reestablished.

Understanding the root causes of these topology changes assists professionals to identify potential issues within their networks more effectively and take necessary precautions.

For instance, consider a scenario where an Ethernet cable connecting two switches gets accidentally unplugged. This would result in Layer 2 connectivity loss between those devices and trigger STP to respond by reconverging on another available path – if one exists – to maintain optimal traffic flow within the network.

Effects on Switch Communication

Topology changes in STP can have a significant impact on switch communication. When a topology change occurs, STP immediately starts the process of recalculating the new path to the root bridge, which results in all ports being placed in a blocking state before they are transitioned into forwarding mode.

Furthermore, if there is regular occurrence of topology changes due to faulty hardware or misconfigured links, then it can lead to Broadcast Storms that further hamper switch communication by overwhelming ports with too much traffic.

Therefore, understanding how to minimize these effects and prevent network failure is essential for successful configuration and maintenance of STP-based networks.

Types and Handling of Topology Changes in STP

There are two types of topology changes in STP - direct and indirect, and understanding how to handle them through configuration and monitoring techniques can ensure a stable network infrastructure.

Direct and Indirect Changes

STP topology changes can be either direct or indirect. Direct changes occur when a link status change is detected on an interface of a switch, which triggers STP to recalculate the path to the root bridge.

This type of topology change is common when a switch port fails due to a physical cable disconnect or connectivity issues.

Indirect changes, on the other hand, happen when there's no actual failure but still affects network traffic flow. Indirect changes occur mainly due to switches not receiving BPDUs from downstream switches in time because they're busy processing data packets or sending their own BPDUs out of order.

An example is when a congested link between two switches causes one of them to stop sending its BPDUs; subsequently, the upstream switch considers it as being down and recalculates STP paths based on this new condition.

Overall knowledge about direct and indirect topology changes in STP enables engineers and administrators to understand how their network behaves during failures and achieve better fault-tolerance by optimizing redundancy protocols like RSTP, which improve convergence times by reducing propagation delays caused by TCN floods at each stage of spanning tree algorithm calculation for VLANs with large numbers of ports or complex topologies involving different layer 2 networks segments spanning multiple sites interconnected via WAN links.

Configuration and Topology Change Notification (TCN) BPDUs

STP relies on Bridge Protocol Data Units (BPDUs) to detect and prevent bridging loops within a local area network. Configuration BPDUs carry information about the switch’s own configuration, while TCN BDPUs are used to signal changes in the network topology.

When a port transitions from blocking to forwarding state, STP sends out a TCN BPDU to inform all other switches of the change.

Root bridge Election and STP Convergence

The root bridge is an STP designated switch that determines the lowest path cost to connect all switches in a network. When a topology change occurs, STP elects a new root bridge based on the lowest path cost. The process of selecting the root bridge is known as the root bridge election process.

Here are some key points about the root bridge election and STP convergence −

  • Root Bridge Election − In a network, when switches initially start up or detect topology changes, they send out Bridge Protocol Data Units (BPDUs) to other connected switches to determine which switch has the lowest path cost to reach the root bridge. The switch with the lowest path cost becomes the root bridge for that segment of the network.

  • Impact on Network Traffic − If there is a significant change in link state or an interruption in communication between switches, then it may cause network convergence. During this time, all Ethernet frames must be re-routed across different paths, which can lead to delays in transmission and affect overall network performance.

  • Rapid Spanning Tree Protocol (RSTP) − RSTP reduces convergence times by reacting faster than traditional STP protocols by making use of port states like "Discarding" and "Learning" instead of just blocking ports outright.

  • Best Practices − It's essential to follow best practices like optimizing network design and implementing portfast features to minimize topology changes' impact on STP convergence.

  • Troubleshooting − When troubleshooting topology changes or convergence issues, it's crucial to check logs and examine network topology using debug commands.

  • Impact on Redundancy Protocols − Topology changes can also affect redundancy protocols such as link aggregation and increase broadcast storms due to high rates of TCN BPDUs sent across multiple ports.

Monitoring and Troubleshooting Topology Changes in STP

To ensure a stable network infrastructure, it's vital to monitor and troubleshoot STP topology changes by regularly checking the status and statistics of STP, using debug commands, and identifying any potential issues that may arise.

Checking STP Status and Statistics

To ensure that the Spanning Tree Protocol (STP) is functioning correctly and to troubleshoot any issues, it is important to check its status and statistics regularly. Here are some steps to follow when checking STP status and statistics −

  • Access the command line interface (CLI) of the switch.

  • Enter "show spanning-tree" or "show spanning-tree detail" command to view the current STP status and topology information. This displays details such as root bridge ID, port roles (root, designated, or blocked), and port state (forwarding, blocking).

  • Look for any inconsistencies in the reported topology, such as ports that should be forwarding but are instead blocking or a non-optimal path being used between nodes.

  • Check the values of key STP timers such as Hello Time, Max Age, and Forward Delay to ensure they are within acceptable limits.

  • Review log files for messages related to STP events such as topology changes or convergence issues.

  • Use debug commands like "debug spanning-tree events" to track down specific issues.

Using Debug Commands to Identify Issues and Resolving them

Debug commands are a useful tool for identifying issues in STP topology changes and resolving them. Here are some steps to follow when using debug commands −

  • Verify that debugging is enabled for the appropriate protocol, such as STP or RSTP.

  • Use the "show spanning-tree" command to view information about the network topology, including bridge IDs, root ports, and designated ports.

  • Use the "debug spanning-tree events" command to track events related to STP topology changes, such as port state transitions and topology change notifications.

  • Use the "debug spanning-tree packets" command to capture and analyze BPDU messages between switches.

  • Review switch logs for any errors or warnings related to STP topology changes.

  • If necessary, use the "clear spannning-tree" command to reset the STP database on a switch.

  • Once you have identified any issues with STP topology changes, take steps to resolve them promptly, such as correcting port configurations or replacing faulty hardware.


Topology changes in Spanning Tree Protocol (STP) can greatly impact network performance and stability. Understanding the different types of topology changes and how STP reacts to them is crucial for effective network troubleshooting.

By monitoring STP status and statistics, using debugging commands to identify issues, and implementing best practices such as portfast, network engineers can minimize the occurrence of topology changes and maintain a reliable infrastructure.

Updated on: 12-May-2023


Kickstart Your Career

Get certified by completing the course

Get Started