Configure Spanning Tree Protocol (STP) on a Catalyst Switch that runs Cisco IOS Software
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Introduction
This document provides instructions on how to configure the Spanning
Tree Protocol (STP) on a Catalyst switch that runs Cisco IOS Software.
Spanning Tree Protocol (STP) is a Layer 2 protocol that runs on bridges
and switches. The specification for STP is IEEE 802.1D. Spanning tree allows a
network design to include spare (redundant) links to provide automatic backup
paths if an active link fails, without the danger of bridge loops, or the need
for manual enabling/disabling of these backup links. The main purpose of STP is
to ensure that you do not create loops when you have redundant paths in your
network. Loops are deadly to a network and can consume all available CPU
resources and bandwidth.
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Requirements
To perform the steps described in this document, you need to have this
equipment:
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Connect your PC to the Catalyst Switch
Follow these steps to connect your PC to the Catalyst Switch:
Complete these steps:
-
Connect a PC to the switch with a console cable.
-
Create a HyperTerminal connection to your switch. For more
information, refer to
Create a
HyperTerminal Connection.
-
Log into the switch with the login and password that you entered in
fields B10 and B11 of the switch Worksheet.
Username:admin
Password:
Note: If you do not know the password for your switch, refer to
Manually
Reset the Password on a Catalyst Switch.
-
Type enable and press Enter to
access the privileged mode. Type the enable password that you entered in field
S5 of the Switch Port Assignment Worksheet.
switch>enable
Password:
switch#
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Configure Spanning Tree Protocol
This section explains how to configure the Spanning Tree Protocol on a
Catalyst Switch.
Configure the Spanning-Tree Mode
The switch supports three spanning-tree modes: PVST+, rapid PVST+, or
MSTP. By default, the switch runs the PVST+ protocol. For more information on
modes refer to
Prepare
to Configure Spanning Tree Protocol on a Catalyst Switch.
Follow these steps to change the spanning-tree mode if you want to
enable a mode that is different from the default mode of PVS
T+.
-
Type configure terminal and press
Enter to enter the switch configuration mode.
switch#configure terminal
switch(config)#
-
Type spanning-tree mode {pvst | mst | rapid-pvst}
and press Enter to configure a spanning-tree mode. In this
example, PVST is selected which enables PVST+ spanning-tree mode.
switch(config)#spanning-tree mode pvst
Note: Configuring rapid-pvst mode requires minimal extra configuration
steps. Follow steps 3 to 5 to configure rapid-pvst mode else go to step
6.
-
Type spanning-tree mode rapid-pvst and press
Enter to enable rapid PVST+.
switch(config)#spanning-tree mode rapid-pvst
-
Type interface interface-id and press
Enter. Valid interfaces include physical ports, VLANs, and
port channels. The VLAN ID range is 1 to 4094. The port-channel range is 1 to
12.
Switch(config)#interface fastEthernet 0/2
Switch(config-if)#
-
Type spanning-tree link-type point-to-point and
press Enter to specify the link type for this port is
point-to-point.
Switch(config-if)#spanning-tree link-type point-to-point
Note: If you connect this port (local port) to a remote port through a
point-to-point link and the local port becomes a designated port, the switch
negotiates with the remote port and rapidly transitions the local port to the
forwarding state.
-
Type end and press Enter to
return to privileged EXEC mode.
Switch(config-if)#end
Switch#
-
If any port on the switch is connected to a port on a legacy 802.1D
switch, restart the protocol migration process on the entire switch. Type
clear spanning-tree detected-protocols and press
Enter.
Switch#clear spanning-tree detected-protocols
-
Type show spanning-tree summary and press
Enter to verify your configuration.
Switch #show spanning-tree summary
Switch is in rapid-pvst mode
Root bridge for: VLAN0001
Extended system ID is enabled
Portfast Default is disabled
PortFast BPDU Guard Default is disabled
Portfast BPDU Filter Default is disabled
Loopguard Default is disabled
EtherChannel misconfig guard is enabled
UplinkFast is disabled
BackboneFast is disabled
Configured Pathcost method used is short
Name Blocking Listening Learning Forwarding STP Active
--------------- -------- --------- -------- ---------- ---------
VLAN0001 0 0 0 20 20
--------------- -------- --------- -------- ---------- ---------
1 vlan 0 0 0 20 20
Configure the Root Switch
The switch maintains a separate spanning-tree instance for each active
VLAN configured on it. A bridge ID, consists the switch priority and the switch
MAC address, is associated with each instance. For each VLAN, the switch with
the lowest bridge ID becomes the root switch for that VLAN.
Follow these steps to configure the Root switch.
-
Type configure terminal and press
Enter to enter the switch configuration mode.
switch#configure terminal
switch(config)#
-
Type spanning-tree vlan 20 root primary and press
Enter to modify the Default VLAN switch priority from the
default value (32768) to a significantly lower value.
Note: The switch software checks the switch priority of the other root
switches for each VLAN and sets its own priority for the specified VLAN to
24576 if this value would cause this switch to become the root for the
specified VLAN.
switch(config)#spanning-tree vlan 20 root primary
Note: The spanning-tree vlan vlan-id root global configuration command
fails if the value necessary to be the root switch is less than 1.
-
Type spanning-tree vlan 21 root primary and press
Enter to modify the Network Management VLAN switch priority
from the default value (32768) to a significantly lower value.
switch(config)#spanning-tree vlan 21 root primary
-
Type spanning-tree vlan 22 root primary and press
Enter to modify the Secure Server VLAN switch priority from
the default value (32768) to a significantly lower value. The switch software
checks the switch priority of the other root switches for each VLAN and sets
its own priority for the specified VLAN to 24576 if this value could cause this
switch to become the root for the specified VLAN.
switch(config)#spanning-tree vlan 22 root primary
-
Type spanning-tree vlan 23 root primary and press
Enter to modify the Guest VLAN switch priority from the
default value (32768) to a significantly lower value. The switch software
checks the switch priority of the other root switches for each VLAN and sets
its own priority for the specified VLAN to 24576 if this value would cause this
switch to become the root for the specified VLAN.
switch(config)#spanning-tree vlan 23 root primary
Note: The spanning-tree vlan vlan-id root global configuration command
fails if the value necessary to be the root switch is less than 1.
-
Type end and press Enter.
switch(config)#end
switch#
-
Type write memory and press
Enter.
switch#write memory
Configuring a Secondary Root Switch
When you configure a Catalyst switch as the secondary root, the switch
priority is modified from the default value (32768) to 28672. The switch is
then likely to become the root switch for the specified VLAN if the primary
root switch fails. This makes us assume that the other network switches use the
default switch priority of 32768 and therefore are unlikely to become the root
switch.
You can execute this command on more than one switch to configure
multiple backup root switches. Use the same network diameter and hello-time
values that you used when you configured the primary root switch with the
spanning-tree vlan vlan-id root primary global configuration
command.
-
Log in to the privilege mode of the alternate switch which you want
it to become secondary root switch for a specific VLAN.
-
Type configure terminal and press
Enter to enter the switch configuration mode.
switch#configure terminal
switch(config)#
-
Type spanning-tree vlan 20 root secondary and
press Enter to modify the Default VLAN switch priority from
the default value 32768 to 28672. The switch software checks the switch
priority of the other root switches for each VLAN and sets its own priority for
the specified VLAN to 24576 if this value would cause this switch to become the
root for the specified VLAN.
switch(config)#spanning-tree vlan 20 root secondary
-
Type spanning-tree vlan 21 root secondary and
press Enter to modify the Network Management VLAN switch
priority from the default value 32768 to 28672. The switch software checks the
switch priority of the other root switches for each VLAN and sets its own
priority for the specified VLAN to 24576 if this value would cause this switch
to become the root for the specified VLAN.
switch(config)#spanning-tree vlan 21 root secondary
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Type spanning-tree vlan 22 root secondary and
press Enter to modify the Secure Server VLAN switch priority
from the default value 32768 to 28672. The switch software checks the switch
priority of the other root switches for each VLAN and sets its own priority for
the specified VLAN to 24576 if this value would cause this switch to become the
root for the specified VLAN.
switch(config)#spanning-tree vlan 22 root secondary
-
Type spanning-tree vlan 23 root secondary and
press Enter to modify the Guest VLAN switch priority from the
default value 32768 to 28672. The switch software checks the switch priority of
the other root switches for each VLAN and sets its own priority for the
specified VLAN to 24576 if this value would cause this switch to become the
root for the specified VLAN.
switch(config)#spanning-tree vlan 23 root secondary
-
Type end and press Enter.
switch(config)#end
switch#
-
Type write memory and press
Enter.
switch#write memory
Configure Port Fast
Spanning-tree Port Fast causes a spanning-tree port to enter the
forwarding state immediately, bypassing the listening and learning states. The
Port Fast feature must be used, only when connecting a single end station or a
server to an access or trunk port. Follow these steps to enable Port
Fast:
-
Type configure terminal and press
Enter to enter the switch configuration mode.
switch#configure terminal
switch(config)#
-
Specify the interface on which Port Fast must be enabled using
interface command.
switch(config)#interface fastEthernet 0/1
-
Type command spanning-tree portfast to enable Port
Fast feature on this interface. If the interface is a trunk interface
then use command spanning-tree portfast trunk to enable Port
Fast feature. Make sure that there are no loops in the network between the
trunk port and the workstation or server before enabling Port Fast on a trunk
port.
switch(config-if)#spanning-tree portfast
-
Type end and press Enter to
return to privileged EXEC mode.
switch(config-if)#end
-
Type write memory and press
Enter.
switch#write memory
To disable Port Fast on a particular interface get into the interface
using interface command and then use the command spanning-tree portfast
disable.
switch(config)#interface fastEthernet 0/1
switch(config-if)#spanning-tree portfast disable
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Disabling Spanning Tree
STP is enabled by default on VLAN 1 and on all newly created VLANs up
to the spanning-tree limit.
Caution: Disable STP only if you are sure there are no loops in the network
topology. When STP is disabled and loops are present in the topology, excessive
traffic and indefinite packet duplication can drastically reduce network
performance.
Follow these steps to disable STP:
-
Type configure terminal and press
Enter to enter the switch configuration mode.
switch#configure terminal
switch(config)#
-
Type no spanning-tree vlan vlan-id and press
Enter to Disable STP on a per-VLAN basis. For vlan-id, the
range is 1 to 4094. Do not enter leading zeros.
switch(config)#no spanning-tree vlan 22
-
Type end and press Enter.
switch(config)#end
switch#
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Next Step
You have completed this procedure.
To make further changes to your switch, refer to the
Switch
Support Page.
To configure other devices in your network, refer to the
Configuration
Overview Page.
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Related Information
