Controlling Spanning Tree

Spanning tree is a Layer 2 protocol used to prevent loops within the network. Several flavors of spanning tree exhibit different characteristics and require special attention.

The use of spanning tree as a method for high availability is a controversial issue, but years of experience troubleshooting spanning-tree problems and the difficulties associated with it determine that it is best to avoid using spanning tree as a mechanism for failover, especially given the complexities with running Per VLAN Spanning-Tree (PVST), Per VLAN Spanning-Tree Plus (PVST+), Rapid Per VLAN Spanning-Tree Plus (RPVST+), Multiple Spanning-Tree Protocol (MSTP), 801.d, and so on, and the interoperability issues that you may face. You should consider using Layer 3 connections and using a routing protocol for better control of traffic and significantly superior tools for troubleshooting your network infrastructure. Don't turn spanning tree off, but use it as an insurance policy in the event of a physical or logical network misconfiguration.

If you must use spanning tree for failover, be sure to understand, document, and appropriately configure the devices within your infrastructure to best utilize the forwarding interfaces of spanning tree. Other mechanisms that should also be employed to minimize any ill effects that spanning tree may cause are the following:

• Loop guard, which performs additional bridge protocol data unit (BPDU) checks and will place a port in an inconsistent state in the event of BPDU detection of the root from a root port or alternative root port.

• BPDU guard, which detects incoming BPDUs and will disable the port.

• Root guard, which checks incoming BPDUs and if they are superior to the existing root, will place the port in an "inconsistent" state.

• Controlling VLANs on specific ports minimizes the impact of spanning tree over switches that may not need to participate in that specific VLAN. Consequently, this will reduce the size of the spanning-tree domain and make the infrastructure easier to manage.

If possible, use Layer 3 routing protocols for high availability. When spanning tree is the only alternative mechanism, be sure you understand and control your network infrastructure to minimize any adverse effects.

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