Catalyst 2940 Switch Software Configuration Guide, 12.1(19)EA1
Configuring STP
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Configuring STP

Table Of Contents

Configuring STP

Understanding Spanning-Tree Features

STP Overview

Spanning-Tree Topology and BPDUs

Bridge ID, Switch Priority, and Extended System ID

Spanning-Tree Interface States

Blocking State

Listening State

Learning State

Forwarding State

Disabled State

How a Switch or Port Becomes the Root Switch or Root Port

Spanning Tree and Redundant Connectivity

Spanning-Tree Address Management

Accelerated Aging to Retain Connectivity

Spanning-Tree Modes and Protocols

Supported Spanning-Tree Instances

Spanning-Tree Interoperability and Backward Compatibility

STP and IEEE 802.1Q Trunks

Configuring Spanning-Tree Features

Default Spanning-Tree Configuration

STP Configuration Guidelines

Disabling Spanning Tree

Configuring the Root Switch

Configuring a Secondary Root Switch

Configuring the Port Priority

Configuring the Path Cost

Configuring the Switch Priority of a VLAN

Configuring Spanning-Tree Timers

Configuring the Hello Time

Configuring the Forwarding-Delay Time for a VLAN

Configuring the Maximum-Aging Time for a VLAN

Displaying the Spanning-Tree Status


Configuring STP


This chapter describes how to configure the Spanning Tree Protocol (STP) on your Catalyst 2940 switch.

For information about optional spanning-tree features, see "Configuring Optional Spanning-Tree Features."


Note For complete syntax and usage information for the commands used in this chapter, refer to the command reference for this release.


This chapter consists of these sections:

Understanding Spanning-Tree Features

Configuring Spanning-Tree Features

Displaying the Spanning-Tree Status

Understanding Spanning-Tree Features

These sections describe how spanning-tree features work:

STP Overview

Spanning-Tree Topology and BPDUs

Bridge ID, Switch Priority, and Extended System ID

Spanning-Tree Interface States

How a Switch or Port Becomes the Root Switch or Root Port

Spanning Tree and Redundant Connectivity

Spanning-Tree Address Management

Accelerated Aging to Retain Connectivity

Spanning-Tree Modes and Protocols

Supported Spanning-Tree Instances

Spanning-Tree Interoperability and Backward Compatibility

STP and IEEE 802.1Q Trunks

For configuration information, see the "Configuring Spanning-Tree Features" section.

For information about optional spanning-tree features, see "Configuring Optional Spanning-Tree Features."

STP Overview

STP is a Layer 2 link management protocol that provides path redundancy while preventing loops in the network. For a Layer 2 Ethernet network to function properly, only one active path can exist between any two stations. Multiple active paths among end stations cause loops in the network. If a loop exists in the network, end stations might receive duplicate messages. Switches might also learn end-station MAC addresses on multiple Layer 2 interfaces. These conditions result in an unstable network. Spanning-tree operation is transparent to end stations, which cannot detect whether they are connected to a single LAN segment or a switched LAN of multiple segments.

The STP uses a spanning-tree algorithm to select one switch of a redundantly connected network as the root of the spanning tree. The algorithm calculates the best loop-free path through a switched Layer 2 network by assigning a role to each port based on the role of the port in the active topology:

Root—A forwarding port elected for the spanning-tree topology

Designated—A forwarding port elected for every switched LAN segment

Alternate—A blocked port providing an alternate path to the root port in the spanning tree

Backup—A blocked port in a loopback configuration

Switches that have ports with these assigned roles are called root or designated switches.

Spanning tree forces redundant data paths into a standby (blocked) state. If a network segment in the spanning tree fails and a redundant path exists, the spanning-tree algorithm recalculates the spanning-tree topology and activates the standby path. Switches send and receive spanning-tree frames, called bridge protocol data units (BPDUs), at regular intervals. The switches do not forward these frames but use them to construct a loop-free path. BPDUs contain information about the sending switch and its ports, including switch and MAC addresses, switch priority, port priority, and path cost. Spanning tree uses this information to elect the root switch and root port for the switched network and the root port and designated port for each switched segment.

When two interfaces on a switch are part of a loop, the spanning-tree port priority and path cost settings determine which interface is put in the forwarding state and which is put in the blocking state. The spanning-tree port priority value represents the location of an interface in the network topology and how well it is located to pass traffic. The path cost value represents the media speed.

Spanning-Tree Topology and BPDUs

The stable, active spanning-tree topology of a switched network is determined by these elements:

The unique bridge ID (switch priority and MAC address) associated with each VLAN on each switch

The spanning-tree path cost to the root switch

The port identifier (port priority and MAC address) associated with each Layer 2 interface

When the switches in a network are powered up, each functions as the root switch. Each switch sends a configuration BPDU through all of its ports. The BPDUs communicate and compute the spanning-tree topology. Each configuration BPDU contains this information:

The unique bridge ID of the switch that the sending switch identifies as the root switch

The spanning-tree path cost to the root

The bridge ID of the sending switch

Message age

The identifier of the sending interface

Values for the hello, forward-delay, and max-age protocol timers

When a switch receives a configuration BPDU that contains superior information (lower bridge ID, lower path cost, and so forth), it stores the information for that port. If this BPDU is received on the root port of the switch, the switch also forwards it with an updated message to all attached LANs for which it is the designated switch.

If a switch receives a configuration BPDU that contains inferior information to that currently stored for that port, it discards the BPDU. If the switch is a designated switch for the LAN from which the inferior BPDU was received, it sends that LAN a BPDU containing the up-to-date information stored for that port. In this way, inferior information is discarded, and superior information is propagated on the network.

A BPDU exchange results in these actions:

One switch in the network is elected as the root switch (the logical center of the spanning-tree topology in a switched network).

For each VLAN, the switch with the highest switch priority (the lowest numerical priority value) is elected as the root switch. If all switches are configured with the default priority (32768), the switch with the lowest MAC address in the VLAN becomes the root switch. The switch priority value occupies the most significant bits of the bridge ID, as shown in Table 11-1.

A root port is selected for each switch (except the root switch). This port provides the best path (lowest cost) when the switch forwards packets to the root switch.

The shortest distance to the root switch is calculated for each switch based on the path cost.

A designated switch for each LAN segment is selected. The designated switch incurs the lowest path cost when forwarding packets from that LAN to the root switch. The port through which the designated switch is attached to the LAN is called the designated port.

Interfaces included in the spanning-tree instance are selected. Root ports and designated ports are put in the forwarding state.

All paths that are not needed to reach the root switch from anywhere in the switched network are placed in the spanning-tree blocking mode.

Bridge ID, Switch Priority, and Extended System ID

The IEEE 802.1D standard requires that each switch has an unique bridge identifier (bridge ID), which determines the selection of the root switch. Because each VLAN is considered as a different logical bridge with PVST+, the same switch must have as many different bridge IDs as VLANs configured on it. Each VLAN on the switch has a unique 8-byte bridge ID; the two most-significant bytes are used for the switch priority, and the remaining six bytes are derived from the switch MAC address.

The Catalyst 2940 switch supports the 802.1T spanning-tree extensions. Some of the bits previously used for the switch priority are now used as the VLAN identifier. The result is that fewer MAC addresses are reserved for the switch, and a larger range of VLAN IDs can be supported, all while maintaining the uniqueness of the bridge ID. As shown in Table 11-1, the two bytes previously used for the switch priority are reallocated into a 4-bit priority value and a 12-bit extended system ID value equal to the VLAN ID.

Table 11-1 Switch Priority Value and Extended System ID

Switch Priority Value
Extended System ID (Set Equal to the VLAN ID)
Bit 16
Bit 15
Bit 14
Bit 13
Bit 12
Bit 11
Bit 10
Bit 9
Bit 8
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1

32768

16384

8192

4096

2048

1024

512

256

128

64

32

16

8

4

2

1


Spanning tree uses the extended system ID, the switch priority, and the allocated spanning-tree MAC address to make the bridge ID unique for each VLAN. With earlier releases, spanning tree used one MAC address per VLAN to make the bridge ID unique for each VLAN.

Support for the extended system ID affects how you manually configure the root switch, the secondary root switch, and the switch priority of a VLAN. For example, when you change the switch priority value, you change the probability that the switch will be elected as the root switch. Configuring a higher value decreases the probability; a lower value increases the probability. For more information, see the "Configuring the Root Switch" section, the "Configuring a Secondary Root Switch" section, and the "Configuring the Switch Priority of a VLAN" section.

Spanning-Tree Interface States

Propagation delays can occur when protocol information passes through a switched LAN. As a result, topology changes can take place at different times and at different places in a switched network. When an interface transitions directly from nonparticipation in the spanning-tree topology to the forwarding state, it can create temporary data loops. Interfaces must wait for new topology information to propagate through the switched LAN before starting to forward frames. They must allow the frame lifetime to expire for forwarded frames that have used the old topology.

Each Layer 2 interface on a switch using spanning tree exists in one of these states:

Blocking—The interface does not participate in frame forwarding.

Listening—The first transitional state after the blocking state when the spanning tree determines that the interface should participate in frame forwarding.

Learning—The interface prepares to participate in frame forwarding.

Forwarding—The interface forwards frames.

Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on the port, or no spanning-tree instance running on the port.

An interface moves through these states:

From initialization to blocking

From blocking to listening or to disabled

From listening to learning or to disabled

From learning to forwarding or to disabled

From forwarding to disabled

Figure 11-1 illustrates how an interface moves through the states.

Figure 11-1 Spanning-Tree Interface States

When you power up the switch, spanning tree is enabled by default, and every interface in the switch, VLAN, or network goes through the blocking state and the transitory states of listening and learning. Spanning tree stabilizes each interface at the forwarding or blocking state.

When the spanning-tree algorithm places a Layer 2 interface in the forwarding state, this process occurs:

1. The interface is in the listening state while spanning tree waits for protocol information to transition the interface to the blocking state.

2. While spanning tree waits the forward-delay timer to expire, it moves the interface to the learning state and resets the forward-delay timer.

3. In the learning state, the interface continues to block frame forwarding as the switch learns end-station location information for the forwarding database.

4. When the forward-delay timer expires, spanning tree moves the interface to the forwarding state, where both learning and frame forwarding are enabled.

Blocking State

A Layer 2 interface in the blocking state does not participate in frame forwarding. After initialization, a BPDU is sent to each interface in the switch. A switch initially functions as the root until it exchanges BPDUs with other switches. This exchange establishes which switch in the network is the root or root switch. If there is only one switch in the network, no exchange occurs, the forward-delay timer expires, and the interfaces move to the listening state. An interface always enters the blocking state after switch initialization.

An interface in the blocking state performs as follows:

Discards frames received on the port

Discards frames switched from another interface for forwarding

Does not learn addresses

Receives BPDUs

Listening State

The listening state is the first state a Layer 2 interface enters after the blocking state. The interface enters this state when the spanning tree determines that the interface should participate in frame forwarding.

An interface in the listening state performs as follows:

Discards frames received on the port

Discards frames switched from another interface for forwarding

Does not learn addresses

Receives BPDUs

Learning State

A Layer 2 interface in the learning state prepares to participate in frame forwarding. The interface enters the learning state from the listening state.

An interface in the learning state performs as follows:

Discards frames received on the port

Discards frames switched from another interface for forwarding

Learns addresses

Receives BPDUs

Forwarding State

A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwarding state from the learning state.

An interface in the forwarding state performs as follows:

Receives and forwards frames received on the port

Forwards frames switched from another port

Learns addresses

Receives BPDUs

Disabled State

A Layer 2 interface in the disabled state does not participate in frame forwarding or in the spanning tree. An interface in the disabled state is nonoperational.

A disabled interface performs as follows:

Discards frames received on the port

Discards frames switched from another interface for forwarding

Does not learn addresses

Does not receive BPDUs

How a Switch or Port Becomes the Root Switch or Root Port

If all switches in a network are enabled with default spanning-tree settings, the switch with the lowest MAC address becomes the root switch. In Figure 11-2, Switch A is elected as the root switch because the switch priority of all the switches is set to the default (32768) and Switch A has the lowest MAC address. However, because of traffic patterns, number of forwarding interfaces, or link types, Switch A might not be the ideal root switch. By increasing the priority (lowering the numerical value) of the ideal switch so that it becomes the root switch, you force a spanning-tree recalculation to form a new topology with the ideal switch as the root.

Figure 11-2 Spanning-Tree Topology

When the spanning-tree topology is calculated based on default parameters, the path between source and destination end stations in a switched network might not be ideal. For instance, connecting higher-speed links to an interface that has a higher number than the root port can cause a root-port change. The goal is to make the fastest link the root port.

For example, assume that one port on Switch B is a Gigabit Ethernet link and that another port on Switch B (a 10/100 link) is the root port. Network traffic might be more efficient over the Gigabit Ethernet link. By changing the spanning-tree port priority on the Gigabit Ethernet interface to a higher priority (lower numerical value) than the root port, the Gigabit Ethernet interface becomes the new root port.

Spanning Tree and Redundant Connectivity

You can create a redundant backbone with spanning tree by connecting two switch interfaces to another device or to two different devices. Spanning tree automatically disables one interface but enables it if the other one fails, as shown in Figure 11-3. If one link is high-speed and the other is low-speed, the low-speed link is always disabled. If the speeds are the same, the port priority and port ID are added together, and spanning tree disables the link with the lowest value.

Figure 11-3 Spanning Tree and Redundant Connectivity

You can also create redundant links between switches by using EtherChannel groups. For more information, see "Configuring EtherChannels."

Spanning-Tree Address Management

IEEE 802.1D specifies 17 multicast addresses, ranging from 0x00180C2000000 to 0x0180C2000010, to be used by different bridge protocols. These addresses are static addresses that cannot be removed.

Regardless of the spanning-tree state, the switch receives but does not forward packets destined for addresses between 0x0180C2000000 and 0x0180C200000F.

If spanning tree is enabled, the switch CPU receives packets destined for 0x0180C2000000 and 0x0180C2000010. If spanning-tree is disabled, the switch forwards those packets as unknown multicast addresses.

Accelerated Aging to Retain Connectivity

The default for aging dynamic addresses is 5 minutes, the default setting of the mac-address-table aging-time global configuration command. However, a spanning-tree reconfiguration can cause many station locations to change. Because these stations could be unreachable for 5 minutes or more during a reconfiguration, the address-aging time is accelerated so that station addresses can be dropped from the address table and then relearned. The accelerated aging is the same as the forward-delay parameter value (spanning-tree vlan vlan-id forward-time seconds global configuration command) when the spanning tree reconfigures.

Because each VLAN is a separate spanning-tree instance, the switch accelerates aging on a per-VLAN basis. A spanning-tree reconfiguration on one VLAN can cause the dynamic addresses learned on that VLAN to be subject to accelerated aging. Dynamic addresses on other VLANs can be unaffected and remain subject to the aging interval entered for the switch.

Spanning-Tree Modes and Protocols

The switch supports PVST+. This spanning-tree mode is based on the IEEE 802.1D standard and Cisco proprietary extensions. It is the default spanning-tree mode used on all Ethernet, Fast Ethernet, and Gigabit Ethernet port-based VLANs. The PVST+ runs on each VLAN on the switch up to the maximum supported, ensuring that each has a loop-free path through the network.

The PVST+ provides Layer 2 load balancing for the VLAN on which it runs. You can create different logical topologies by using the VLANs on your network to ensure that all of your links are used but that no one link is oversubscribed. Each instance of PVST+ on a VLAN has a single root switch. This root switch propagates the spanning-tree information associated with that VLAN to all other switches in the network. Because each switch has the same information about the network, this process ensures that the network topology is maintained.

Supported Spanning-Tree Instances

In PVST+, the switch supports up to 4 spanning-tree instances.

For information about how spanning tree interoperates with the VLAN Trunking Protocol (VTP), see the "STP Configuration Guidelines" section.

Spanning-Tree Interoperability and Backward Compatibility

Table 11-2 lists the interoperability and compatibility among the supported spanning-tree modes in a network.

Table 11-2 PVST+, MSTP, and Rapid-PVST+ Interoperability

 
PVST+
MSTP
Rapid PVST+

PVST+

Yes

Yes (with restrictions)

Yes (reverts to PVST+)

MSTP

Yes (with restrictions)

Yes

Yes (reverts to PVST+)

Rapid PVST+

Yes (reverts to PVST+)

Yes (reverts to PVST+)

Yes


In a mixed Multiple STP (MSTP) and PVST+ network, the common spanning-tree (CST) root must be inside the MST backbone, and a PVST+ switch cannot connect to multiple MST regions.

When a network contains switches running rapid PVST+ and switches running PVST+, we recommend that the rapid-PVST+ switches and PVST+ switches be configured for different spanning-tree instances. In the rapid-PVST+ spanning-tree instances, the root switch must be a rapid-PVST+ switch. In the PVST+ instances, the root switch must be a PVST+ switch. The PVST+ switches should be at the edge of the network.

The Catalyst 2940 switch does not support MSTP or rapid PVST+.

STP and IEEE 802.1Q Trunks

The IEEE 802.1Q standard for VLAN trunks imposes some limitations on the spanning-tree strategy for a network. The standard requires only one spanning-tree instance for all VLANs allowed on the trunks. However, in a network of Cisco switches connected through 802.1Q trunks, the switches maintain one spanning-tree instance for each VLAN allowed on the trunks.

When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch uses PVST+ to provide spanning-tree interoperability. If rapid PVST+ is enabled, the switch uses it instead of PVST+. The switch combines the spanning-tree instance of the 802.1Q VLAN of the trunk with the spanning-tree instance of the non-Cisco 802.1Q switch.

However, all PVST+ or rapid-PVST+ information is maintained by Cisco switches separated by a cloud of non-Cisco 802.1Q switches. The non-Cisco 802.1Q cloud separating the Cisco switches is treated as a single trunk link between the switches.

The external spanning-tree behavior on access ports and trunk ports is not affected by PVST+ or rapid PVST+.

For more information on 802.1Q trunks, see "Configuring VLANs."

Configuring Spanning-Tree Features

These sections describe how to configure spanning-tree features:

Default Spanning-Tree Configuration

STP Configuration Guidelines

Disabling Spanning Tree

Configuring the Root Switch

Configuring a Secondary Root Switch

Configuring the Port Priority

Configuring the Path Cost

Configuring the Switch Priority of a VLAN

Configuring Spanning-Tree Timers

Configuring the Hello Time

Configuring the Forwarding-Delay Time for a VLAN

Configuring the Maximum-Aging Time for a VLAN

Default Spanning-Tree Configuration

Table 11-3 shows the default spanning-tree configuration.

Table 11-3 Default Spanning-Tree Configuration 

Feature
Default Setting

Enable state

Enabled on VLAN 1.

UP to 4 spanning-tree instances can be enabled.

Spanning-tree mode

PVST+. (Rapid PVST+ and MSTP are disabled.)

Switch priority

32768.

Spanning-tree port priority (configurable on a per-interface basis)

128.

Spanning-tree port cost (configurable on a per-interface basis)

1000 Mbps: 4.

100 Mbps: 19.

10 Mbps: 100.

Spanning-tree VLAN port priority (configurable on a per-VLAN basis)

128.

Spanning-tree VLAN port cost (configurable on a per-VLAN basis)

1000 Mbps: 4.

100 Mbps: 19.

10 Mbps: 100.

Spanning-tree timers

Hello time: 2 seconds.

Forward-delay time: 15 seconds.

Maximum-aging time: 20 seconds.


STP Configuration Guidelines

You can disable STP on one of the VLANs and then enable it on the VLAN where you want it to run. Use the no spanning-tree vlan vlan-id global configuration command to disable spanning-tree on a specific VLAN, and use the spanning-tree vlan vlan-id global configuration command to enable spanning-tree on the desired VLAN.


Caution Switches that are not running spanning tree still forward BPDUs that they receive so that the other switches on the VLAN that have a running spanning-tree instance can break loops. Therefore, spanning tree must be running on enough switches to break all the loops in the network; for example, at least one switch on each loop in the VLAN must be running spanning tree. It is not absolutely necessary to run spanning tree on all switches in the VLAN; however, if you are running spanning tree only on a minimal set of switches, an incautious change to the network that introduces another loop into the VLAN can result in a broadcast storm.

Spanning-tree commands determine the configuration of VLAN spanning-tree instances. You create a spanning-tree instance when you assign an interface to a VLAN. The spanning-tree instance is removed when the last interface is moved to another VLAN. You can configure switch and port parameters before a spanning-tree instance is created; these parameters are applied when the spanning-tree instance is created.

Disabling Spanning Tree

Spanning-tree is enabled by default on VLAN 1 and on all newly created VLANs up to the spanning-tree limit specified in the "Supported Spanning-Tree Instances" section. Disable STP only if you are sure there are no loops in the network topology.


Caution When spanning tree is disabled and loops are present in the topology, excessive traffic and indefinite packet duplication can drastically reduce network performance.

Beginning in privileged EXEC mode, follow these steps to disable STP on a per-VLAN basis:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

no spanning-tree vlan vlan-id

Disable STP on a per-VLAN basis.

For vlan-id, you can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma. The range is 1 to 1005.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show spanning-tree vlan vlan-id

Verify your entries.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To re-enable spanning tree, use the spanning-tree vlan vlan-id global configuration command.

Configuring the Root Switch

The switch maintains a separate spanning-tree instance for each active VLAN configured on it. A bridge ID, consisting of 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.

To configure a switch to become the root for the specified VLAN, use the spanning-tree vlan vlan-id root global configuration command to modify the switch priority from the default value (32768) to a significantly lower value. When you enter this command, the switch checks the switch priority of the root switches for each VLAN. Because of the extended system ID support, the switch sets its own priority for the specified VLAN to 24576 if this value will cause this switch to become the root for the specified VLAN.

If any root switch for the specified VLAN has a switch priority lower than 24576, the switch sets its own priority for the specified VLAN to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit switch priority value as shown in Table 11-1.)


Note The spanning-tree vlan vlan-id root global configuration command fails if the necessary value to be the root switch is less than 1.


With the extended system ID, if all network devices in VLAN 20 have the default priority of 32768, entering the spanning-tree vlan 20 root primary command on the switch sets the switch priority to 24576, which causes this switch to become the root switch for VLAN 20.


Note If your network consists of switches that both do and do not support the extended system ID, it is unlikely that the switch with the extended system ID support will become the root switch. The extended system ID increases the switch priority value every time the VLAN number is greater than the priority of the connected switches running older software.



Note The root switch for each spanning-tree instance should be a backbone or distribution switch. Do not configure an access switch as the spanning-tree primary root.


Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.


Note After configuring the switch as the root switch, we recommend that you avoid manually configuring the hello time, forward-delay time, and maximum-age time by using the spanning-tree vlan vlan-id hello-time, spanning-tree vlan vlan-id forward-time, and the spanning-tree vlan vlan-id max-age global configuration commands.


Beginning in privileged EXEC mode, follow these steps to configure a switch to become the root for the specified VLAN:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

spanning-tree vlan vlan-id root primary [diameter net-diameter [hello-time seconds]]

Configure a switch to become the root for the specified VLAN.

For vlan-id, you can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma. The range is 1 to 1005.

(Optional) For diameter net-diameter, specify the maximum number of switches between any two end stations. The range is 2 to 7.

(Optional) For hello-time seconds, specify the interval in seconds between the generation of configuration messages by the root switch. The range is 1 to 10 seconds; the default is 2 seconds.

Note When you enter this command without the optional keywords, the switch recalculates the forward-time, hello-time, max-age, and priority settings. If you had previously configured these parameters, the switch overrides and recalculates them.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show spanning-tree detail

Verify your entries.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree vlan vlan-id root global configuration command.

Configuring a Secondary Root Switch

When you configure a Catalyst 2940 switch that supports the extended system ID 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 is assuming 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 as you used when you configured the primary root switch with the spanning-tree vlan vlan-id root primary global configuration command.

Beginning in privileged EXEC mode, follow these steps to configure a switch to become the secondary root for the specified VLAN. This procedure is optional:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

spanning-tree vlan vlan-id root secondary [diameter net-diameter [hello-time seconds]]

Configure a switch to become the secondary root for the specified VLAN.

For vlan-id, you can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma. The range is 1 to 1005.

(Optional) For diameter net-diameter, specify the maximum number of switches between any two end stations. The range is 2 to 7.

(Optional) For hello-time seconds, specify the interval in seconds between the generation of configuration messages by the root switch. The range is 1 to 10 seconds; the default is 2 seconds.

Use the same network diameter and hello-time values that you used when configuring the primary root switch. See the "Configuring the Root Switch" section.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show spanning-tree detail

Verify your entries.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree vlan vlan-id root global configuration command.

Configuring the Port Priority

If a loop occurs, spanning tree uses the port priority when selecting an interface to put into the forwarding state. You can assign higher priority values (lower numerical values) to interfaces that you want selected first and lower priority values (higher numerical values) that you want selected last. If all interfaces have the same priority value, spanning tree puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.

Beginning in privileged EXEC mode, follow these steps to configure the port priority of an interface. This procedure is optional:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify an interface to configure.

Valid interfaces include physical interfaces and port-channel logical interfaces (port-channel port-channel-number).

Step 3 

spanning-tree port-priority priority

Configure the port priority for an interface.

For priority, the range is 0 to 240 in increments of 16. The default is 128. The lower the number, the higher the priority.

Valid priority values are 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, and 240. All other values are rejected.

Step 4 

spanning-tree vlan vlan-id port-priority priority

Configure the VLAN port priority for an interface.

For vlan-id, you can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma. The range is 1 to 1005.

For priority, the range is 0 to 240 in increments of 16. The default is 128. The lower the number, the higher the priority.

Valid priority values are 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, and 240. All other values are rejected.

Step 5 

end

Return to privileged EXEC mode.

Step 6 

show spanning-tree interface interface-id

or

show spanning-tree vlan vlan-id

Verify your entries.

Step 7 

copy running-config startup-config

(Optional) Save your entries in the configuration file.


Note The show spanning-tree interface interface-id privileged EXEC command displays information only if the port is in a link-up operative state. Otherwise, you can use the show running-config interface privileged EXEC command to confirm the configuration.


To return the interface to its default setting, use the no spanning-tree [vlan vlan-id] port-priority interface configuration command. For information on how to configure load sharing on trunk ports by using spanning-tree port priorities, see the "Load Sharing Using STP" section.

Configuring the Path Cost

The spanning-tree path cost default value is derived from the media speed of an interface. If a loop occurs, spanning tree uses cost when selecting an interface to put in the forwarding state. You can assign lower cost values to interfaces that you want selected first and higher cost values that you want selected last. If all interfaces have the same cost value, spanning tree puts the interface with the lowest interface number in the forwarding state and blocks the other interfaces.

Beginning in privileged EXEC mode, follow these steps to configure the cost of an interface. The switch supports the per-VLAN spanning-tree plus (PVST+) and a maximum of four spanning-tree instances.

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

interface interface-id

Enter interface configuration mode, and specify an interface to configure. Valid interfaces include physical interfaces and port-channel logical interfaces (port-channel port-channel-number).

Step 3 

spanning-tree cost cost

Configure the cost for an interface.

If a loop occurs, spanning tree uses the path cost when selecting an interface to place into the forwarding state. A lower path cost represents higher-speed transmission.

For cost, the range is 1 to 200000000; the default value is derived from the media speed of the interface.

Step 4 

spanning-tree vlan vlan-id cost cost

Configure the cost for a VLAN.

If a loop occurs, spanning tree uses the path cost when selecting an interface to place into the forwarding state. A lower path cost represents higher-speed transmission.

For vlan-id, you can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma. The range is 1 to 1005.

For cost, the range is 1 to 200000000; the default value is derived from the media speed of the interface.

Step 5 

end

Return to privileged EXEC mode.

Step 6 

show spanning-tree interface interface-id

or

show spanning-tree vlan vlan-id

Verify your entries.

Step 7 

copy running-config startup-config

(Optional) Save your entries in the configuration file.


Note The show spanning-tree interface interface-id privileged EXEC command displays information only for ports that are in a link-up operative state. Otherwise, you can use the show running-config privileged EXEC command to confirm the configuration.


To return the interface to its default setting, use the no spanning-tree [vlan vlan-id] cost interface configuration command. For information on how to configure load sharing on trunk ports by using spanning-tree path costs, see the "Load Sharing Using STP" section.

Configuring the Switch Priority of a VLAN

You can configure the switch priority and make it more likely that the switch will be chosen as the root switch.


Note Exercise care when using this command. For most situations, we recommend that you use the spanning-tree vlan vlan-id root primary and the spanning-tree vlan vlan-id root secondary global configuration commands to modify the switch priority.


Beginning in privileged EXEC mode, follow these steps to configure the switch priority of a VLAN. This procedure is optional:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

spanning-tree vlan vlan-id priority priority

Configure the switch priority of a VLAN.

For vlan-id, you can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma. The range is 1 to 1005.

For priority, the range is 0 to 61440 in increments of 4096; the default is 32768. The lower the number, the more likely the switch will be chosen as the root switch.

Valid priority values are 4096, 8192, 12288, 16384, 20480, 24576, 28672, 32768, 36864, 40960, 45056, 49152, 53248, 57344, and 61440. All other values are rejected.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show spanning-tree vlan vlan-id

Verify your entries.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree vlan vlan-id priority global configuration command.

Configuring Spanning-Tree Timers

Table 11-4 describes the timers that affect the entire spanning-tree performance.

Table 11-4 Spanning-Tree Timers 

Variable
Description

Hello timer

Determines how often the switch broadcasts hello messages to other switches.

Forward-delay timer

Determines how long each of the listening and learning states last before the interface begins forwarding.

Maximum-age timer

Determines the amount of time the switch stores protocol information received on an interface.


The sections that follow provide the configuration steps.

Configuring the Hello Time

You can configure the interval between the generation of configuration messages by the root switch by changing the hello time.


Note Exercise care when using this command. For most situations, we recommend that you use the spanning-tree vlan vlan-id root primary and the spanning-tree vlan vlan-id root secondary global configuration commands to modify the hello time.


Beginning in privileged EXEC mode, follow these steps to configure the hello time of a VLAN. This procedure is optional:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

spanning-tree vlan vlan-id hello-time seconds

Configure the hello time of a VLAN. The hello time is the interval between the generation of configuration messages by the root switch. These messages mean that the switch is alive.

For vlan-id, you can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma. The range is 1 to 1005.

For seconds, the range is 1 to 10; the default is 2.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show spanning-tree vlan vlan-id

Verify your entries.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree vlan vlan-id hello-time global configuration command.

Configuring the Forwarding-Delay Time for a VLAN

Beginning in privileged EXEC mode, follow these steps to configure the forwarding-delay time for a VLAN. This procedure is optional. This procedure is optional:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

spanning-tree vlan vlan-id forward-time seconds

Configure the forward time of a VLAN. The forward delay is the number of seconds a port waits before changing from its spanning-tree learning and listening states to the forwarding state.

For vlan-id, you can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma. The range is 1 to 1005.

For seconds, the range is 4 to 30; the default is 15.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show spanning-tree vlan vlan-id

Verify your entries.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree vlan vlan-id forward-time global configuration command.

Configuring the Maximum-Aging Time for a VLAN

Beginning in privileged EXEC mode, follow these steps to configure the maximum-aging time for a VLAN:

 
Command
Purpose

Step 1 

configure terminal

Enter global configuration mode.

Step 2 

spanning-tree vlan vlan-id max-age seconds

Configure the maximum-aging time of a VLAN. The maximum-aging time is the number of seconds a switch waits without receiving spanning-tree configuration messages before attempting a reconfiguration.

For vlan-id, you can specify a single VLAN identified by VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma. The range is 1 to 1005.

For seconds, the range is 6 to 40; the default is 20.

Step 3 

end

Return to privileged EXEC mode.

Step 4 

show spanning-tree vlan vlan-id

Verify your entries.

Step 5 

copy running-config startup-config

(Optional) Save your entries in the configuration file.

To return the switch to its default setting, use the no spanning-tree vlan vlan-id max-age global configuration command.

Displaying the Spanning-Tree Status

To display the spanning-tree status, use one or more of the privileged EXEC commands in Table 11-5:

Table 11-5 Commands for Displaying Spanning-Tree Status 

Command
Purpose

show spanning-tree active

Displays spanning-tree information on active interfaces only.

show spanning-tree detail

Displays a detailed summary of interface information.

show spanning-tree interface interface-id

Displays spanning-tree information for the specified interface.

show spanning-tree summary [totals]

Displays a summary of port states or displays the total lines of the STP state section.


You can clear spanning-tree counters by using the clear spanning-tree [interface interface-id] privileged EXEC command.

For information about other keywords for the show spanning-tree privileged EXEC command, refer to the command reference for this release.