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Spanning Tree Protocol Features

Spanning Tree Protocol (STP) resolves redundant topologies into loop-free, treelike topologies. When switches are interconnected via multiple paths, STP prevents loops from being formed. An STP loop (or forwarding loops) can occur when the entire network fails because of a hardware failure, a configuration issue, or a network attack. STP loops can be costly, causing major network outages. The following STP features can be used to improve the stability of the Layer 2 networks.

Bridge Protocol Data Unit (BPDU) Guard

Bridge protocol data units (BPDU) are data messages exchanged between bridges using spanning tree protocol to detect loops in a network topology. BPDU contains management and control data information that is used to determine the root bridge and establish the port roles—for example: root, designated, or blocked port.

The BPDU Guard feature is designed to keep the active topology predictable and to enhance switch network reliability by enforcing the STP domain borders.

The guard can be enabled globally on the switch or enabled on a per-interface basis. In a valid configuration, ports with port fast enabled do not receive BPDUs. Receiving a BPDU on a port with port fast enabled signals an invalid configuration, such as the connection of an unauthorized device, and the BPDU Guard feature puts the interface in the error-disabled state.

At the global level, BPDU Guard can be enabled on a port with port fast enabled using the spanning-tree portfast bpduguard default global configuration command. Spanning tree shuts down interfaces that are in a port fast operational state.

At the interface level, BPDU Guard can be enabled on an interface by using the spanning-tree bpduguard enable interface configuration command without also enabling the port fast feature. When the interface receives a BPDU, the switch assumes that a problem exists and puts the interface in the error-disabled state.

The BPDU Guard feature provides a secure response to invalid configurations because you must manually put the interface back in service. In a service-provider network environment, the BPUD Guard feature can be used to prevent an access port from participating in the spanning tree.

Root Guard

In a switched network environment with shared administrative control or in a service provider (SP) environment where there are many connections to other switches (into customer networks), it is important to identify the correct placement of the root bridge. If possible, it is also important to identify a specific predetermined location to achieve an optimal forwarding loop-free topology. There is no mechanism in the standard STP to enforce the position of the root bridge, as any bridge in a network with a lower bridge ID can assume the role of the root bridge. Sometimes because of a misconfiguration, a spanning tree may converge incorrectly by selecting an imprecise switch to be the root switch. This situation can be prevented by enabling the Root Guard feature. For example, you could enable Root Guard on SP-side switch interfaces that connect to a customer-side switch. With the Root Guard feature implemented, if a switch outside the SP network becomes the root switch, the interface is put in a blocked state, and spanning tree will select a new root switch. The customer's switch does not become the root switch and is not in the path to the root.

With the Root Guard feature, a Layer 2 interface is set as the designated port, and if any device through this port becomes the root bridge, the interface is placed into the blocked (root-inconsistent) state. The Root Guard feature can be enabled by using the spanning-tree guard root command in interface configuration mode.

EtherChannel Guard

The EtherChannel Guard feature is used to detect EtherChannel misconfigurations between the switch and a connected device. An example of a misconfiguration is when the channel parameters are not identical and do not match on both sides of the EtherChannel. Another example could be when only one side is configured with channel parameters. EtherChannel parameters must be the same on both sides for the guard to work.

When the switch detects an EtherChannel misconfiguration, the EtherChannel Guard places the switch interface in the error-disabled state and displays an error message.

The EtherChannel Guard feature can be enabled by using the spanning-tree etherchannel guard misconfig global configuration command.

Loop Guard

The Loop Guard feature provides an additional layer of protection against the Layer 2 forwarding loops (STP loops) by preventing alternative or root ports from becoming designated ports because of a failure resulting in a unidirectional link. This feature works best when enabled on all switches across a network. By default, the spanning tree does not send BPDUs on root or alternative ports.

The Loop Guard feature can be enabled by using the spanning-tree loopguard default global configuration command.

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