9 Common Spanning Tree Mistakes

Frequent spanning tree protocol misconfigurations cause network problems

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Ethernet devices running the Spanning Tree Protocol (STP) have been implemented in networks since the early 1990s. Many organizations take STP for granted and do not configure it per industry best practices. STP errors are very common and during the past 15 years we have witnessed the same errors being made over-and-over again. For such a well established protocol, it is surprising that we have not progressed beyond these types of STP configuration mistakes. This article covers the most frequent STP errors and how to correct them.

The IEEE 802.1D Spanning Tree Protocol (STP) was invented by Radia Pearlman in 1985 when working at Digital Equipment Corporation (DEC). STP is a layer-2 protocol that runs between bridges to help create a loop-free network topology. Bridge Protocol Data Units (BPDUs) are packets sent between Ethernet switches (essentially multi-port bridges) to elect a root bridge, calculate the best path to the root and block any ports that create loops. The resulting tree, with the root at the top, spans all bridges in the LAN, hence the name: spanning tree. If you want to understand STP you should read the Algorhyme poem by Radia.

Spanning tree works efficiently at preventing loops with the default configuration settings. Thus, many people forget to adjust any parameters and simply accept the defaults. This leads many people to ignore STP in their network designs and, after many years, organizations are surprised to discover they have network issues related to spanning tree. There have been many optimizations to STP, but, if they have not been configured, the network is not benefiting from these new features.

Spanning Tree Problems

This is picture of a typically misconfigured spanning tree environment that illustrates many of the common mistakes that are covered in this article.

1 - No Root Bridge Configured

Many organizations take spanning tree for granted and simply accept the default configuration settings. This leaves all switches in the environment using the default root bridge priority of 32768. If all switches have the same root bridge priority, the switch with the lowest MAC address will be elected as the root bridge. Many networks have not been configured with a single switch to have a lower root bridge priority which would force that core switch to be elected as the STP root for any or all VLANs. In this situation, it is possible that a small access-layer switch with a low MAC address could be the STP root. This situation would add some performance overhead and make for longer convergence times because of the root bridge reelection.

As seen in the above picture, the switch that is the STP root is actually core Switch 2 because it happens to have a lower MAC address than core Switch 1.

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