Spanning Tree

Spanning Tree Protocol (STP) is a Layer 2 loop prevention mechanism that ensures a loop-free topology in Ethernet networks by selectively blocking redundant paths while maintaining network connectivity and providing automatic failover capabilities. It supports multiple implementations including Per-VLAN Spanning Tree Plus (PVST+), Rapid Per-VLAN Spanning Tree Plus (RPVST+), and Multiple Spanning Tree (MST) protocol, each offering different approaches to VLAN-aware loop prevention and convergence optimization. Spanning Tree configuration includes features like PortFast for edge ports, BPDU Guard for security, root bridge selection, and path cost manipulation to optimize network topology and ensure rapid recovery from link failures while preventing broadcast storms and network instability.

Diagram

Classes

configuration (iosxe.devices)

Name	Type	Constraint	Mandatory	Default Value
spanning_tree	Class	`[spanning_tree]`	No

spanning_tree (iosxe.devices.configuration)

Name	Type	Constraint	Mandatory
mode	Choice	`mst`, `pvst`, `rapid-pvst`	No
loopguard_default	Boolean	`true`, `false`	No
portfast_default	Boolean	`true`, `false`	No
portfast_bpduguard_default	Boolean	`true`, `false`	No
extend_system_id	Boolean	`true`, `false`	No
mst_instances	List	`[mst_instances]`	No

mst_instances (iosxe.devices.configuration.spanning_tree)

Name	Type	Constraint	Mandatory	Default Value
id	Integer	min: `0`, max: `4094`	Yes
vlan_ids	List	Integer	No

By preventing Layer 2 loops, Spanning Tree Protocol (STP) ensures network stability and rapid recovery from link failures.

Spanning Tree Parameters

Key Components:

STP Mode (mode): Selects the spanning tree protocol (PVST+, Rapid-PVST+, MST).
Bridge Priority (priority): Sets the priority for root bridge election.
VLANs (vlan_ids): Specifies VLANs for PVST+/Rapid-PVST+ or maps VLANs to MST instances.
MST Instance ID and VLAN Mappings (mst_instances): Configures MST instances and their associated VLANs.
Port Priority (port_priority): Sets the priority for individual ports.
Path Cost (path_cost): Adjusts the cost for spanning tree path selection.
PortFast (portfast_default, portfast): Enables PortFast globally or per interface.
BPDU Guard (portfast_bpduguard_default, bpduguard): Enables BPDU Guard globally or per interface.
BPDU Filter (bpdufilter): Enables BPDU Filter globally or per interface.
Root Guard (rootguard): Protects the root bridge role.
Loop Guard (loopguard_default, loopguard): Prevents loops caused by unidirectional links.
EtherChannel Guard (etherchannel_guard): Protects EtherChannel links.
Max Age, Forward Delay, Hello Time (max_age, forward_delay, hello_time): Sets STP timers.
Extend System ID (extend_system_id): Enables extended system ID for bridge priority.

Key Parameters Briefly Explained:

mode: STP protocol selection.
priority: Bridge priority.
vlan_ids: VLANs for STP.
mst_instances: MST instance configuration.
port_priority, path_cost: Port-specific settings.
portfast_default, portfast: PortFast settings.
portfast_bpduguard_default, bpduguard: BPDU Guard settings.
bpdufilter, rootguard, loopguard_default, loopguard, etherchannel_guard: Protective features.
max_age, forward_delay, hello_time: STP timers.
extend_system_id: Extended system ID.

You can use these Spanning Tree parameters to control loop prevention, optimize convergence, and enhance the security of your Layer 2 network. Customize the STP mode, bridge priorities, port settings, and protective features to fit your network’s topology, performance requirements, and resilience needs. Adjusting these parameters lets you tailor how your network handles redundancy and prevents loops.

Sample Configuration

The following configuration describes how to set up Spanning Tree Protocol on a Cisco IOS-XE device, including STP mode, global PortFast and BPDU Guard, MST configuration, and instance mapping for robust Layer 2 loop prevention and optimized convergence.

spanning-tree mode rapid-pvst
spanning-tree loopguard default
spanning-tree portfast default
spanning-tree portfast bpduguard default
spanning-tree extend system-id
!
spanning-tree vlan 1,10,20,30 root primary
spanning-tree vlan 100,200,300 root secondary
!
spanning-tree mst configuration
 name MY_MST_REGION
 revision 1
 instance 0 vlan 1,10,20,30
 instance 1 vlan 100,200,300
 instance 2 vlan 400,500,600
 exit
!
spanning-tree mst 0 priority 4096
spanning-tree mst 1 priority 8192
!
interface GigabitEthernet0/1
 spanning-tree portfast
 spanning-tree bpduguard enable

Example YAML Code

The following YAML code defines Spanning Tree Protocol configuration on an IOS-XE device, including global settings for mode, loopguard, portfast, BPDU guard, and extend system ID, with an example for MST instance configuration.

iosxe:
  devices:
    - name: Device1
      configuration:
        spanning_tree:
          mode: rapid-pvst
          loopguard_default: true
          portfast_default: true
          portfast_bpduguard_default: true
          extend_system_id: true
    - name: Device2
      configuration:
        spanning_tree:
          mode: mst
          loopguard_default: true
          portfast_default: false
          portfast_bpduguard_default: true
          extend_system_id: true
          mst_instances:
            - id: 0
              vlan_ids: [1, 10, 20, 30]
            - id: 1
              vlan_ids: [100, 200, 300]
            - id: 2
              vlan_ids: [400, 500, 600]