Enabling and configuring STP on an interface

Follow these steps to enable STP and STP features on an interface.

Before you begin

Globally enable STP and STP parameters.

About this task

The parameters can be configured individually by:

Entering the commands in steps 1-3
Running the relevant parameter command
Verifying the result
Saving the configuration

For detailed descriptions of the parameters and features, see the sections STP parameters and STP features.

Procedure

Enter global configuration mode.
```
device# configure terminal
```

Enter interface configuration mode.

device(config)# interface ethernet 0/20

Enable the interface.
```
device(conf-if-eth-0/20)# no shutdown
```
Configure the path cost for spanning tree calculations on the interface.
```
device(conf-if-eth-0/20)# spanning-tree cost 10000
```
The lower the path cost means a greater chance that the interface becomes the root port. The range is 1 through 200000000. The default path cost is assigned as per the port speed.
Enable BPDU guard on the interface.
```
device(conf-if-eth-0/20)# spanning-tree port-fast bpdu-guard
```
BPDU guard removes a node that reflects BPDUs back in the network. It enforces the STP domain borders and keeps the active topology predictable by not allowing any network devices behind a BPDU guard-enabled port to participate in STP.
Configure Root Guard on the interface.
```
device(conf-if-eth-0/20)# spanning-tree guard root
```
Root Guard protects the root bridge from malicious attacks and unintentional misconfigurations where a bridge device that is not intended to be the root bridge becomes the root bridge.
Specify an interface link-type.
```
device(conf-if-eth-0/20)# spanning-tree link-type point-to-point
```
Specifying a point-to-point link enables rapid spanning tree transitions to the forwarding state. Specifying a shared link disables spanning tree rapid transitions. The default setting is point-to-point.
Specify port priority to influence the selection of root or designated ports.
```
device(conf-if-eth-0/20)# spanning-tree priority 64
```
The range is from 0 through 240 in increments of 16. The default value is 128.

Verify the configuration.

  
device# show spanning-tree brief

 Spanning-tree Mode: Spanning Tree Protocol

      Root ID      Priority 4096
                   Address 768e.f805.5800
                   Hello Time 8, Max Age 25, Forward Delay 20

      Bridge ID    Priority 4096
                   Address 768e.f805.5800
                   Hello Time 8, Max Age 25, Forward Delay 20

 Interface    Role  Sts  Cost       Prio  Link-type     Edge
 ------------------------------------------------------------
 Eth 0/2      DES   FWD  2000       128   P2P            No
 Eth 0/20     DES   FWD  1000        64   P2P            No
 Eth 0/25     DIS   DIS  20000000   128   P2P            No
 Eth 0/30     DIS   DIS  20000000   128   P2P            No
 Eth 0/31     DIS   DIS  2000000    128   P2P            No

Note

Observe that the settings comply with the formula set out in the STP parameters section, as:

(2 × (forward delay - 1)) ≥ maximum age ≥ (2 × (hello time + 1))

or in this case: 38 ≥ 25 ≥ 18

device# show running-config interface ethernet 0/20 
interface ethernet 0/20 
switchport
switchport mode access
switchport access val 1
spanning-tree cost 1000
spanning-tree guard root
spanning-tree link-type point-to-point
spanning-tree portfast bpdu-guard 
spanning-tree priority 64

Save the settings by copying the running configuration to the startup configuration.
```
device# copy running-config startup-config
```

STP on an interface configuration example

device# configure terminal
device(config)# interface ethernet 0/20
device(conf-if-eth-0/20)# no shutdown
device(conf-if-eth-0/20)# spanning-tree cost 10000
device(conf-if-eth-0/20)# spanning-tree port-fast bpdu-guard
device(conf-if-eth-0/20)# spanning-tree guard root
device(conf-if-eth-0/20)# spanning-tree link-type point-to-point
device(conf-if-eth-0/20)# spanning-tree priority 64
device(conf-if-eth-0/20)# end
device# show spanning-tree brief
device# copy running-config startup-config