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Spanning TreeGlobal Config

spanning-tree mode [rapid-pvst|mst]

Configures the Spanning Tree Protocol (STP) mode to either Rapid PVST+ or MST on a Cisco Nexus switch.

Definition: spanning-tree mode [rapid-pvst|mst] is a Cisco NX-OS global config command. Configures the Spanning Tree Protocol (STP) mode to either Rapid PVST+ or MST on a Cisco Nexus switch.

Overview

The 'spanning-tree mode' command is used to select the Spanning Tree Protocol (STP) variant on Cisco Nexus switches. STP is a Layer 2 protocol that prevents loops in Ethernet networks by creating a loop-free logical topology. Cisco NX-OS supports two modes: Rapid PVST+ (Per-VLAN Spanning Tree Plus) and MST (Multiple Spanning Tree).

Rapid PVST+ runs a separate instance of Rapid Spanning Tree Protocol (RSTP) for each VLAN, providing fast convergence (typically 1-2 seconds) and per-VLAN load balancing. It is the default mode on Nexus switches and is suitable for networks with a moderate number of VLANs. MST, based on IEEE 802.1s, allows mapping multiple VLANs to a single spanning-tree instance, reducing CPU and bandwidth overhead in large networks with many VLANs.

MST requires configuration of a region (name, revision number, and VLAN-to-instance mapping) that must be consistent across all switches in the region. The choice between Rapid PVST+ and MST depends on network size, scalability requirements, and interoperability with other vendors. In troubleshooting workflows, verifying the STP mode is a first step when diagnosing STP-related issues like loops or convergence problems.

On Nexus, the command is applied globally and takes effect immediately, but a topology change may occur. Always ensure that all switches in the Layer 2 domain use the same STP mode to avoid incompatibility.

Syntax·Global Config
spanning-tree mode {rapid-pvst | mst}

When to Use This Command

  • Enabling Rapid PVST+ for fast convergence in a VLAN-based STP topology.
  • Configuring MST to reduce STP instances in a large Layer 2 network with many VLANs.
  • Migrating from PVST+ to Rapid PVST+ to improve convergence time.
  • Setting MST to map multiple VLANs to a single spanning-tree instance for scalability.

Parameters

ParameterSyntaxDescription
rapid-pvstrapid-pvstSelects Rapid Per-VLAN Spanning Tree Plus mode. This is the default on Nexus switches. It runs RSTP per VLAN, providing fast convergence and per-VLAN load balancing.
mstmstSelects Multiple Spanning Tree mode (IEEE 802.1s). Allows mapping multiple VLANs to a single spanning-tree instance. Requires additional MST region configuration.

Command Examples

Enable Rapid PVST+ mode

spanning-tree mode rapid-pvst

This command sets the STP mode to Rapid PVST+. No output is displayed if successful. Use 'show spanning-tree' to verify.

Enable MST mode

spanning-tree mode mst

This command sets the STP mode to MST. After this, you must configure MST region parameters (name, revision, VLAN-to-instance mapping).

Understanding the Output

The 'spanning-tree mode' command does not produce output on its own. To verify the current STP mode, use 'show spanning-tree' or 'show running-config | include spanning-tree'. In the output of 'show spanning-tree', the first line indicates the mode, e.g., 'Spanning tree enabled protocol rstp' for Rapid PVST+ or 'Spanning tree enabled protocol mst' for MST.

For MST, additional details like region name, revision, and instance mappings appear. A healthy state shows all ports in forwarding/blocking as expected; problem states include inconsistent port roles or BPDU guard violations.

Configuration Scenarios

Configuring Rapid PVST+ on a Nexus Switch

A small data center with 50 VLANs. Need fast convergence and per-VLAN load balancing.

Topology

Nexus-1 --- Nexus-2 | | +--- Access Switches

Steps

  1. 1.Enter global configuration mode.
  2. 2.Set STP mode to rapid-pvst.
  3. 3.Optionally configure STP priorities per VLAN for load balancing.
Configuration
! Enter global config
configure terminal
! Set STP mode
spanning-tree mode rapid-pvst
! Optional: set root bridge for VLAN 10
spanning-tree vlan 10 root primary

Verify: Use 'show spanning-tree' to verify mode and root bridge status.

Watch out: Rapid PVST+ is default; if already set, no change occurs. Ensure all switches in the network use the same mode.

Configuring MST on a Nexus Switch

A large campus network with 500 VLANs. Need to reduce STP instances.

Topology

Nexus-Core --- Nexus-Dist-1 --- Access | Nexus-Dist-2

Steps

  1. 1.Enter global configuration mode.
  2. 2.Set STP mode to mst.
  3. 3.Enter MST configuration submode.
  4. 4.Set region name and revision number.
  5. 5.Map VLANs to instances.
  6. 6.Exit and apply.
Configuration
! Enter global config
configure terminal
! Set STP mode to MST
spanning-tree mode mst
! Enter MST configuration
spanning-tree mst configuration
! Set region name
name CAMPUS
! Set revision
revision 1
! Map VLANs 1-100 to instance 1
instance 1 vlan 1-100
! Map VLANs 101-200 to instance 2
instance 2 vlan 101-200
! Exit
end

Verify: Use 'show spanning-tree mst configuration' to verify region settings and 'show spanning-tree mst instance 1' to see instance details.

Watch out: MST region parameters must match exactly on all switches in the region; otherwise, they will be considered separate regions.

Troubleshooting with This Command

When troubleshooting STP issues on Cisco Nexus, the 'spanning-tree mode' command is a starting point to confirm the STP variant in use. Use 'show spanning-tree' to display the current mode and overall STP state. For Rapid PVST+, check per-VLAN root bridge and port roles.

For MST, verify region consistency with 'show spanning-tree mst configuration'. Common issues include: (1) Inconsistent STP mode across switches – ensure all switches in the Layer 2 domain use the same mode. (2) MST region mismatch – verify region name, revision, and VLAN mapping match on all switches. (3) Unexpected root bridge election – check bridge priorities. (4) Port role inconsistencies – look for ports in 'alternate' or 'backup' roles that should be 'blocking'. Use 'show spanning-tree vlan <vlan>' for Rapid PVST+ or 'show spanning-tree mst instance <instance>' for MST to drill down.

If BPDU guard or root guard is enabled, check for err-disabled ports. For convergence issues, verify that all links are operating in full duplex and that STP timers are not misconfigured. On Nexus, the default STP mode is Rapid PVST+, which provides fast convergence; if MST is used, ensure that the network is designed with MST regions properly.

Use 'debug spanning-tree' cautiously in a lab to trace BPDU exchanges. Always check the system logs for STP-related messages.

CCNA Exam Tips

1.

CCNP SWITCH: Know that Rapid PVST+ is the default on Nexus switches and provides faster convergence than PVST+.

2.

CCNP SWITCH: Understand that MST requires consistent region configuration across all switches in the MST region.

3.

CCNP SWITCH: Remember that changing STP mode can cause a temporary topology change; plan maintenance windows.

Common Mistakes

Mistake: Forgetting to configure MST region parameters after switching to MST mode. Consequence: MST defaults to the CIST (Common and Internal Spanning Tree) with no VLAN mapping, causing all VLANs to map to instance 0.

Mistake: Using 'spanning-tree mode pvst' (PVST+) on Nexus; Nexus only supports rapid-pvst and mst. Consequence: Command rejected.

Mistake: Changing STP mode without verifying compatibility with neighboring switches. Consequence: STP incompatibility may cause loops or loss of connectivity.

spanning-tree mode [rapid-pvst|mst] vs spanning-tree port type edge

The 'spanning-tree mode' and 'spanning-tree port type edge' commands are often considered together because both influence Spanning Tree Protocol behavior, but at different levels: one sets the global STP algorithm, while the other defines a port's role within that algorithm. Understanding their distinct scopes is essential for proper STP configuration on Cisco NX-OS.

Aspectspanning-tree mode [rapid-pvst|mst]spanning-tree port type edge
ScopeGlobal configurationInterface configuration
Effect on STPSelects protocol variant: Rapid PVST+ or MSTDesignates port as edge (PortFast) to skip listening/learning
PersistenceAffects all VLANs and ports on switchApplies only to the specific interface
Typical UseTo enable fast convergence with RSTP or MSTTo immediately transition access ports to forwarding state
PrecedenceGlobal setting overridden by per-VLAN mode if configuredDoes not change STP mode; works with any mode

Use spanning-tree mode [rapid-pvst|mst] when you need to switch the entire switch's STP algorithm for faster convergence or to support multiple spanning-tree instances.

Use spanning-tree port type edge when you have end-user devices attached and want to eliminate STP delay on that specific port.

Platform Notes

On Cisco NX-OS (Nexus), the 'spanning-tree mode' command differs from Cisco IOS in that Nexus only supports rapid-pvst and mst; it does not support legacy PVST+ or classic STP. The default mode is rapid-pvst. In contrast, Cisco IOS defaults to PVST+ (or rapid-pvst on some platforms).

When migrating from IOS to NX-OS, note that the command syntax is identical but the available modes are limited. For MST configuration, the submode 'spanning-tree mst configuration' is similar to IOS, but Nexus requires explicit 'name' and 'revision' commands. On Nexus, the 'spanning-tree mode' command is applied globally and takes effect immediately; there is no need to reload.

For verification, 'show spanning-tree' output includes the mode in the first line. On other platforms like Arista, the equivalent command is 'spanning-tree mode' with options 'rapid-pvst' or 'mst', but Arista also supports 'rstp' as a mode. On Juniper, STP is configured under 'protocols stp' with 'bridge-mode' for RSTP or 'vlan' for VLAN-based.

Understanding these differences is crucial for multi-vendor environments.

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