You can configure a spanning tree port as an edge port, a network port, or a normal port. A port can be in only one of these states at a given time. The default spanning tree port type is normal.

Edge ports, which are connected to Layer 2 hosts, can be either an access port or a trunk port.

Note	If you configure a port connected to a Layer 2 switch or bridge as an edge port, you might create a bridging loop.

Network ports are connected only to Layer 2 switches or bridges.

Note	If you mistakenly configure ports that are connected to Layer 2 hosts, or edge devices, as spanning tree network ports, those ports will automatically move into the blocking state.

You can use Bridge Assurance to protect against certain problems that can cause bridging loops in the network. Specifically, you use Bridge Assurance to protect against a unidirectional link failure or other software failure and a device that continues to forward data traffic when it is no longer running the spanning tree algorithm.

Note	Bridge Assurance is supported only by Rapid PVST+ and MST.

Bridge Assurance is enabled by default and can only be disabled globally. Also, Bridge Assurance can be enabled only on spanning tree network ports that are point-to-point links. Finally, both ends of the link must have Bridge Assurance enabled. If the device on one side of the link has Bridge Assurance enabled and the device on the other side either does not support Bridge Assurance or does not have this feature enabled, the connecting port is blocked.

With Bridge Assurance enabled, BPDUs are sent out on all operational network ports, including alternate and backup ports, for each hello time period. If the port does not receive a BPDU for a specified period, the port moves into the blocking state and is not used in the root port calculation. Once that port receives a BPDU, it resumes the normal spanning tree transitions.

Figure 1. Network with Normal STP Topology. This figure shows a normal STP topology.

Figure 2. Network Problem without Running Bridge Assurance. This figure demonstrates a potential network problem when the device fails and you are not running Bridge Assurance.

Figure 3. Network STP Topology Running Bridge Assurance. This figure shows the network with Bridge Assurance enabled, and the STP topology progressing normally with bidirectional BPDUs issuing from every STP network port.

Figure 4. Network Problem Averted with Bridge Assurance Enabled. This figure shows how the potential network problem does not happen when you have Bridge Assurance enabled on your network.

Enabling BPDU Guard shuts down that interface if a BPDU is received.

You can configure BPDU Guard at the interface level. When configured at the interface level, BPDU Guard shuts the port down as soon as the port receives a BPDU, regardless of the port type configuration.

When you configure BPDU Guard globally, it is effective only on operational spanning tree edge ports. In a valid configuration, Layer 2 LAN edge interfaces do not receive BPDUs. A BPDU that is received by an edge Layer 2 LAN interface signals an invalid configuration, such as the connection of an unauthorized device. BPDU Guard, when enabled globally, shuts down all spanning tree edge ports when they receive a BPDU.

BPDU Guard provides a secure response to invalid configurations, because you must manually put the Layer 2 LAN interface back in service after an invalid configuration.

Note	When enabled globally, BPDU Guard applies to all operational spanning tree edge interfaces.

You can use BPDU Filtering to prevent the device from sending or even receiving BPDUs on specified ports.

When configured globally, BPDU Filtering applies to all operational spanning tree edge ports. You should connect edge ports only to hosts, which typically drop BPDUs. If an operational spanning tree edge port receives a BPDU, it immediately returns to a normal spanning tree port type and moves through the regular transitions. In that case, BPDU Filtering is disabled on this port, and spanning tree resumes sending BPDUs on this port.

In addition, you can configure BPDU Filtering by the individual interface. When you explicitly configure BPDU Filtering on a port, that port does not send any BPDUs and drops all BPDUs that it receives. You can effectively override the global BPDU Filtering setting on individual ports by configuring the specific interface. This BPDU Filtering command on the interface applies to the entire interface, whether the interface is trunking or not.

Caution

Use care when configuring BPDU Filtering per interface. If you explicitly configure BPDU Filtering on a port that is not connected to a host, it can result in bridging loops because the port will ignore any BPDU that it receives and go to forwarding.

Loop Guard helps prevent bridging loops that could occur because of a unidirectional link failure on a point-to-point link.

An STP loop occurs when a blocking port in a redundant topology erroneously transitions to the forwarding state. Transitions are usually caused by a port in a physically redundant topology (not necessarily the blocking port) that stops receiving BPDUs.

When you enable Loop Guard globally, it is useful only in switched networks where devices are connected by point-to-point links. On a point-to-point link, a designated bridge cannot disappear unless it sends an inferior BPDU or brings the link down. However, you can enable Loop Guard on shared links per interface,

You can use Loop Guard to determine if a root port or an alternate/backup root port receives BPDUs. If the port that was previously receiving BPDUs is no longer receiving BPDUs, Loop Guard puts the port into an inconsistent state (blocking) until the port starts to receive BPDUs again. If such a port receives BPDUs again, the port—and link—is deemed viable again. The protocol removes the loop-inconsistent condition from the port, and the STP determines the port state because the recovery is automatic.

Loop Guard isolates the failure and allows STP to converge to a stable topology without the failed link or bridge. Disabling Loop Guard moves all loop-inconsistent ports to the listening state.

You can enable Loop Guard on a per-port basis. When you enable Loop Guard on a port, it is automatically applied to all of the active instances or VLANs to which that port belongs. When you disable Loop Guard, it is disabled for the specified ports.

Enabling Loop Guard on a root device has no effect but provides protection when a root device becomes a nonroot device.

When you enable Root Guard on a port, Root Guard does not allow that port to become a root port. If a received BPDU triggers an STP convergence that makes that designated port become a root port, that port is put into a root-inconsistent (blocked) state. After the port stops receiving superior BPDUs, the port is unblocked again. Through STP, the port moves to the forwarding state. Recovery is automatic.

When you enable Root Guard on an interface, this functionality applies to all VLANs to which that interface belongs.

You can use Root Guard to enforce the root bridge placement in the network. Root Guard ensures that the port on which Root Guard is enabled is the designated port. Normally, root bridge ports are all designated ports, unless two or more of the ports of the root bridge are connected. If the bridge receives superior BPDUs on a Root Guard-enabled port, the bridge moves this port to a root-inconsistent STP state. In this way, Root Guard enforces the position of the root bridge.

You cannot configure Root Guard globally.

MST interoperates with Rapid PVST+ with no need for user configuration. The PVST simulation feature enables this interoperability.

Note	PVST simulation is enabled by default when you enable MST. By default, all interfaces on the device interoperate between MST and Rapid PVST+.

However, you may want to control the connection between MST and Rapid PVST+ to protect against accidentally connecting an MST-enabled port to a port enabled to run Rapid PVST+. Because Rapid PVST+ is the default STP mode, you may encounter many Rapid PVST+ connections.

Disabling Rapid PVST+ simulation, which can be done per port or globally for the entire device, moves the MST-enabled port to the blocking state once it detects it is connected to a Rapid PVST+-enabled port. This port remains in the inconsistent state until the port stops receiving Rapid PVST+/SSTP BPDUs, and then the port resumes the normal STP transition process.

The root bridge for all STP instances must all be in either the MST region or the Rapid PVST+ side. If the root bridge for all STP instances are not on one side or the other, the software moves the port into a PVST simulation-inconsistent state.

Note	We recommend that you put the root bridge for all STP instances in the MST region.

The software supports high availability for STP. However, the statistics and timers are not restored when STP restarts. The timers start again and the statistics begin from 0.

Note	See the Cisco Nexus 9000 Series NX-OS High Availability and Redundancy Guide, for complete information on high-availability features.

The spanning tree port type designation depends on the device the port is connected to, as follows:

• Edge—Edge ports are connected to Layer 2 hosts and are access ports.

• Network—Network ports are connected only to Layer 2 switches or bridges and can be either access or trunk ports.

• Normal—Normal ports are neither edge ports nor network ports; they are normal spanning tree ports. These ports can be connected to any device.

You can configure the port type either globally or per interface. By default, the spanning tree port type is normal.

You can configure spanning tree edge ports on specified interfaces. Interfaces configured as spanning tree edge ports immediately transition to the forwarding state, without passing through the blocking or learning states, on linkup.

This command has four states:

• spanning-tree port type edge—This command explicitly enables edge behavior on the access port.

• spanning-tree port type edge trunk—This command explicitly enables edge behavior on the trunk port.

Note	If you enter the spanning-tree port type edge trunk command, the port is configured as an edge port even in the access mode.

• spanning-tree port type normal—This command explicitly configures the port as a normal spanning tree port and the immediate transition to the forwarding state is not enabled.

• no spanning-tree port type—This command implicitly enables edge behavior if you define the spanning-tree port type edge default command in global configuration mode. If you do not configure the edge ports globally, the no spanning-tree port type command is equivalent to the spanning-tree port type normal command.

You can configure spanning tree network ports on specified interfaces.

Bridge Assurance runs only on spanning tree network ports.

This command has three states:

• spanning-tree port type network—This command explicitly configures the port as a network port. If you enable Bridge Assurance globally, it automatically runs on a spanning tree network port.

• spanning-tree port type normal —This command explicitly configures the port as a normal spanning tree port and Bridge Assurance cannot run on this interface.

• no spanning-tree port type—This command implicitly enables the port as a spanning tree network port if you define the spanning-tree port type network default command in global configuration mode. If you enable Bridge Assurance globally, it automatically runs on this port.

Note	A port connected to Layer 2 hosts that is configured as a network ports automatically moves into the blocking state.

You can enable BPDU Guard globally by default. In this condition, the system shuts down an edge port that receives a BPDU.

Note	We recommend that you enable BPDU Guard on all edge ports.

You can enable BPDU Guard on specified interfaces. Enabling BPDU Guard shuts down the port if it receives a BPDU.

You can configure BPDU Guard on specified interfaces as follows:

• spanning-tree bpduguard enable —Unconditionally enables BPDU Guard on the interface.

• spanning-tree bpduguard disable —Unconditionally disables BPDU Guard on the interface.

• no spanning-tree bpduguard —Enables BPDU Guard on the interface if it is an operational edge port and if the spanning-tree port type edge bpduguard default command is configured.

You can enable BPDU Filtering globally by default on spanning tree edge ports.

If an edge port with BPDU Filtering enabled receives a BPDU, it loses its operation status as edge port and resumes the regular STP transitions. However, this port maintains its configuration as an edge port.

Caution

Be careful when using this command. Using this command incorrectly can cause bridging loops.

You can apply BPDU Filtering to specified interfaces. When enabled on an interface, that interface does not send any BPDUs and drops all BPDUs that it receives. This BPDU Filtering functionality applies to the entire interface, whether trunking or not.

Caution

Be careful when you enter the spanning-tree bpdufilter enable command on specified interfaces. Explicitly configuring BPDU Filtering on a port that is not connected to a host can result in bridging loops because the port will ignore any BPDU that it receives and go to forwarding.

You can enter this command to override the port configuration on specified interfaces.

This command has three states:

• spanning-tree bpdufilter enable—Unconditionally enables BPDU Filtering on the interface.

• spanning-tree bpdufilter disable—Unconditionally disables BPDU Filtering on the interface.

• no spanning-tree bpdufilter ——Enables BPDU Filtering on the interface if the interface is in operational edge port and if you configure the spanning-tree port type edge bpdufilter default command.

You can enable Loop Guard globally by default on all point-to-point spanning tree normal and network ports. Loop Guard does not run on edge ports.

Loop Guard provides additional security in the bridge network. Loop Guard prevents alternate or root ports from becoming the designated port because of a failure that could lead to a unidirectional link.

Note	Entering the Loop Guard command for the specified interface overrides the global Loop Guard command.

Note	You can run Loop Guard on spanning tree normal or network ports. You can run Root Guard on all spanning tree ports: normal, edge, or network.

You can enable either Loop Guard or Root Guard on specified interfaces.

Enabling Root Guard on a port means that port cannot become a root port, and Loop Guard prevents alternate or root ports from becoming the designated port because of a failure that could lead to a unidirectional link

Both Loop Guard and Root Guard enabled on an interface apply to all VLANs to which that interface belongs.

Note	Entering the Loop Guard command for the specified interface overrides the global Loop Guard command.

Note	PVST simulation is enabled by default. By default, all interfaces on the device interoperate between MST and Rapid PVST+.

MST interoperates with Rapid PVST+. However, to prevent an accidental connection to a device that does not run MST as the default STP mode, you may want to disable this automatic feature. If you disable PVST simulation, the MST-enabled port moves to the blocking state once it detects it is connected to a Rapid PVST+-enabled port. This port remains in the inconsistent state until the port stops receiving BPDUs, and then the port resumes the normal STP transition process.

You can block this automatic feature either globally or per port. You can enter the global command and change the PVST simulation setting for the entire device while you are in interface command mode.

Note	PVST simulation is enabled by default. By default, all interfaces on the device interoperate between MST and Rapid PVST+.

You can configure PVST simulation only when you are running MST on the device (Rapid PVST+ is the default STP mode). MST interoperates with Rapid PVST+. However, to prevent an accidental connection to a device that does not run MST as the default STP mode, you may want to disable this automatic feature. If you disable PVST simulation, the MST-enabled port moves to the blocking state once it detects that it is connected to a Rapid PVST+-enabled port. This port remains in the inconsistent state until the port stops receiving Rapid PVST+ BPDUs, and then the port resumes the normal STP transition process.

You can block this automatic feature either globally or per port.