Spanning Tree Protocol (STP) is a network protocol that prevents loops and ensures redundancy in LAN switching. However, STP also introduces some challenges, such as topology changes and topology change notifications (TCNs). In this article, you will learn how to identify and resolve these issues and improve your STP performance and stability.
A topology change occurs when a link status changes, such as when a port goes up or down, or when a switch joins or leaves the network. A topology change may affect the STP root bridge election, the port roles and states, and the forwarding paths. To adapt to the new topology, STP sends topology change notifications (TCNs) to inform other switches about the change and trigger a recalculation of the spanning tree. TCNs are sent as special BPDU frames from the port that detected the change to the root bridge, and then from the root bridge to all other switches.
In particular, topology changes can trigger IGMP snooping configurations to flood multicast traffic to all ports whilst issuing a membership query to identify the ports required to reach all members. Unexpected or frequent TCN occurrences can be problematic for networks with large amounts of multicast traffic.
One way to identify topology changes and TCNs is to use the show spanning-tree command on your switches. This command displays the current STP status, including the root bridge ID, the port roles and states, the port costs and priorities, and the number of topology changes and TCNs received and sent. You can also use the show spanning-tree detail command to see more information, such as the time since the last topology change, the originator of the last TCN, and the TCN flag on each port. Another way to identify topology changes and TCNs is to use debug commands, such as debug spanning-tree events, debug spanning-tree bpdu, and debug spanning-tree tc. These commands show the real-time messages and events related to STP, including the link status changes, the BPDU frames, and the TCN flags.
To prevent topology changes and TCNs from occurring frequently, you should follow some best practices. Using portfast on access ports that connect to end devices, such as PCs, servers, or printers, allows them to transition directly to the forwarding state, reducing the STP convergence time. Additionally, bpduguard can be enabled on portfast-enabled ports to disable them if they receive any BPDU frames. Root guard can be used on designated ports that connect to other switches to prevent them from becoming root ports if they receive superior BPDUs. Lastly, loop guard can be used on non-designated ports that connect to other switches to prevent them from becoming designated ports if they stop receiving BPDUs. Following these best practices will help ensure network stability and performance.
Monitoring and troubleshooting your STP network regularly is essential to identify and resolve topology changes and TCNs. Show spanning-tree summary can give you a quick overview of the STP status, such as the number of root, designated, alternate, and backup ports, the number of instances and VLANs, and the STP mode and timers. Show spanning-tree inconsistentports can help detect ports in an inconsistent state due to portfast, bpduguard, root guard, or loop guard violations. You should fix these ports with corrections in configuration or wiring errors. Show spanning-tree vlan vlan-id can provide the STP status for a specific VLAN, such as the root bridge ID, the root port, the port roles and states, and the port costs and priorities. Lastly, show spanning-tree mst can show the STP status for multiple spanning tree (MST) instances if you are using MST as your STP mode. This allows you to map multiple VLANs to a single spanning tree instance while reducing BPDUs and TCNs. Make sure your MST configuration and region are correct and consistent across your switches.