Table Of Contents

Configuring STP

Understanding Spanning-Tree Features

STP Overview

Supported Spanning-Tree Instances

Bridge Protocol Data Units

Election of the Root Switch

Bridge ID, Switch Priority, and Extended System ID

Spanning-Tree Timers

Creating the Spanning-Tree Topology

Spanning-Tree Interface States

Blocking State

Listening State

Learning State

Forwarding State

Disabled State

Spanning-Tree Address Management

STP and IEEE 802.1Q Trunks

VLAN-Bridge STP

Spanning Tree and Redundant Connectivity

Accelerated Aging to Retain Connectivity

Configuring Spanning-Tree Features

Default STP Configuration

STP Configuration Guidelines

Disabling STP

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 the Hello Time

Configuring the Forwarding-Delay Time for a VLAN

Configuring the Maximum-Aging Time for a VLAN

Configuring STP for Use in a Cascaded Stack

Displaying Spanning-Tree Status

Configuring STP

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

For information about the Rapid Spanning Tree Protocol (RSTP) and the Multiple Spanning Tree Protocol (MSTP), see "Configuring RSTP and MSTP." For information about optional spanning-tree features, see "Configuring Optional Spanning-Tree Features."

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Note For complete syntax and usage information for the commands used in this chapter, refer to the Catalyst 3550 Multilayer Switch Command Reference for this release.

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This chapter consists of these sections:

•Understanding Spanning-Tree Features

•Configuring Spanning-Tree Features

•Displaying Spanning-Tree Status

Understanding Spanning-Tree Features

This section describes how spanning-tree features work. It includes this information:

•STP Overview

•Supported Spanning-Tree Instances

•Bridge Protocol Data Units

•Election of the Root Switch

•Bridge ID, Switch Priority, and Extended System ID

•Spanning-Tree Timers

•Creating the Spanning-Tree Topology

•Spanning-Tree Interface States

•Spanning-Tree Address Management

•STP and IEEE 802.1Q Trunks

•VLAN-Bridge STP

•Spanning Tree and Redundant Connectivity

•Accelerated Aging to Retain Connectivity

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. 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.

When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a network. The spanning-tree algorithm calculates the best loop-free path throughout a switched Layer 2 network. 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 the frames to construct a loop-free path.

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 defines a tree with a root switch and a loop-free path from the root to all switches in the Layer 2 network. 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.

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 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 media speed.

Supported Spanning-Tree Instances

The switch supports the per-VLAN spanning tree (PVST) and a maximum of 128 spanning-tree instances. For information about how spanning tree interoperates with the VLAN Trunking Protocol (VTP), see the "STP Configuration Guidelines" section.

Bridge Protocol Data Units

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 is elected as the root switch.

•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 interfaces not included in the spanning tree are blocked.

Election of the Root Switch

All switches in the Layer 2 network participating in spanning tree gather information about other switches in the network through an exchange of BPDU data messages. This exchange of messages results in these actions:

•The election of a unique root switch for each spanning-tree instance

•The election of a designated switch for every switched LAN segment

•The removal of loops in the switched network by blocking Layer 2 interfaces connected to redundant links

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.

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.

The root switch is the logical center of the spanning-tree topology in a switched network. 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.

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.

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.

In Release 12.1(8)EA1 and later, Catalyst 3550 switches support the 802.1T spanning-tree extensions, and 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 14-1, 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. In earlier releases, the switch priority is a 16-bit value.

Table 14-1 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 more information, see the "Configuring the Root Switch" section, "Configuring a Secondary Root Switch" section, and "Configuring the Switch Priority of a VLAN" section.

Spanning-Tree Timers

Table 14-2 describes the timers that affect the entire spanning-tree performance.

Table 14-2 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.

Creating the Spanning-Tree Topology

In Figure 14-1, 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 14-1 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 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 14-2 illustrates how an interface moves through the states.

Figure 14-2 Spanning-Tree Interface States

When you power up the switch, STP 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

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 0x1080C200000F.

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

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 per-VLAN spanning tree+ (PVST+) to provide spanning-tree interoperability. It 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+ 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.

PVST+ is automatically enabled on 802.1Q trunks, and no user configuration is required. The external spanning-tree behavior on access ports and Inter-Switch Link (ISL) trunk ports is not affected by PVST+.

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

VLAN-Bridge STP

Cisco VLAN-bridge STP is used with the fallback bridging feature (bridge groups), which forwards non-IP protocols such as DECnet or IPX between two or more VLAN bridge domains or routed ports. The VLAN-bridge STP allows the bridge groups to form a spanning tree on top of the individual VLAN spanning trees to prevent loops from forming if there are multiple connections among VLANs. It also prevents the individual spanning trees from the VLANs being bridged from collapsing into a single spanning tree.

To support VLAN-bridge STP, some of the spanning-tree timers are increased. To use the fallback bridging feature, you must have the enhanced multilayer switch image installed on your switch. For more information, see "Configuring Fallback Bridging."

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 14-3. 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 14-3 Spanning Tree and Redundant Connectivity

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

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.

Configuring Spanning-Tree Features

These sections include spanning-tree configuration information:

•Default STP Configuration

•Disabling STP

•STP Configuration Guidelines

•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 the Hello Time

•Configuring the Forwarding-Delay Time for a VLAN

•Configuring the Maximum-Aging Time for a VLAN

•Configuring STP for Use in a Cascaded Stack

Default STP Configuration

Table 14-3 shows the default STP configuration.

Table 14-3 Default STP Configuration 

Feature	Default Setting
Enable state	Enabled on VLAN 1. Up to 128 spanning-tree instances can be enabled.
Switch priority	32768.
Spanning-tree port priority (configurable on a per-interface basis—used on interfaces configured as Layer 2 access ports)	128.
Spanning-tree port cost (configurable on a per-interface basis—used on interfaces configured as Layer 2 access ports)	1000 Mbps: 4. 100 Mbps: 19. 10 Mbps: 100.
Spanning-tree VLAN port priority (configurable on a per-VLAN basis—used on interfaces configured as Layer 2 trunk ports)	128.
Spanning-tree VLAN port cost (configurable on a per-VLAN basis—used on interfaces configured as Layer 2 trunk ports)	1000 Mbps: 4. 100 Mbps: 19. 10 Mbps: 100.
Hello time	2 seconds.
Forward-delay time	15 seconds.
Maximum-aging time	20 seconds.

STP Configuration Guidelines

If more VLANs are defined in the VTP than there are spanning-tree instances, you can enable STP on only 128 VLANs. The remaining VLANs operate with spanning tree disabled. If the number of VLANs exceeds 128, we recommend that you enable the MSTP to map multiple VLANs to a single spanning-tree instance. For more information, see the "Configuring RSTP and MSTP."

If 128 instances of spanning tree are already in use, 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 STP on a specific VLAN, and use the spanning-tree vlan vlan-id global configuration command to enable STP on the desired VLAN.

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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.

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Note If you have already used all available spanning-tree instances on your switch, adding another VLAN anywhere in the VTP domain creates a VLAN that is not running spanning tree on that switch. If you have the default allowed list on the trunk ports of that switch, the new VLAN is carried on all trunk ports. Depending on the topology of the network, this could create a loop in the new VLAN that will not be broken, particularly if there are several adjacent switches that have all run out of spanning-tree instances. You can prevent this possibility by setting up allowed lists on the trunk ports of switches that have used up their allocation of spanning-tree instances. Setting up allowed lists is not necessary in many cases and can make it more labor-intensive to add another VLAN to the network.

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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 STP

STP is enabled by default on VLAN 1 and on all newly created VLANs up to the spanning-tree limit specified in Table 14-3. Disable STP only if you are sure there are no loops in the network topology.

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

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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, the range is 1 to 4094. Do not enter leading zeros.
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 STP, 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, 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 so that the switch becomes the root switch for the specified VLAN. 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 14-1.)

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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.

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Before Release 12.1(8)EA1, entering the spanning-tree vlan vlan-id root global configuration command on a Catalyst 3550 switch (no extended system ID) caused it to set its own switch priority for the specified VLAN to 8192 if this value caused this switch to become the root for the specified VLAN. If any root switch for the specified VLAN has a switch priority lower than 8192, the switch sets its own priority for the specified VLAN to 1 less than the lowest switch priority.

These examples show the effect of the spanning-tree vlan vlan-id root command with and without the extended system ID support:

•For Catalyst 3550 switches with the extended system ID (Release 12.1(8)EA1 and later), 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.

•For Catalyst 3550 switches without the extended system ID (software earlier than Release 12.1(8)EA1), if all network devices in VLAN 100 have the default priority of 32768, entering the spanning-tree vlan 100 root primary command on the switch sets the switch priority for VLAN 100 to 8192, which causes this switch to become the root switch for VLAN 100.

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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.

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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.

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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.

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Note 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 after configuring the switch as the root switch.

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Beginning in privileged EXEC mode, follow these steps to configure a switch as the root switch:

	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 as the root switch. •For vlan-id, the range is 1 to 4094. Do not enter leading zeros. •(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.
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 3550 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. For Catalyst 3550 switches without the extended system ID support (software earlier than Release 12.1(8)EA1), the switch priority is changed to 16384.

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 as the secondary root switch:

	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 as the secondary root switch. •For vlan-id, the range is 1 to 4094. Do not enter leading zeros. •(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.

Cisco IOS uses the port priority value when the interface is configured as an access port and uses VLAN port priority values when the interface is configured as a trunk port.

Beginning in privileged EXEC mode, follow these steps to configure the port priority of an interface:

	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 that is an access port. For priority, the range is 0 to 255; the default is 128. The lower the number, the higher the priority.
Step 4	spanning-tree vlan vlan-id port-priority priority	Configure the VLAN port priority for an interface that is a trunk port. •For vlan-id, the range is 1 to 4094. Do not enter leading zeros. •For priority, the range is 0 to 255; the default is 128. The lower the number, the higher the priority.
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.

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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.

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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.

Spanning tree uses the cost value when the interface is configured as an access port and uses VLAN port cost values when the interface is configured as a trunk port.

Beginning in privileged EXEC mode, follow these steps to configure the cost of an interface:

	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 that is an access port. 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 VLAN cost for an interface that is a trunk port. 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, the range is 1 to 4094. Do not enter leading zeros. •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.

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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.

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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 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.

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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.

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Beginning in privileged EXEC mode, follow these steps to configure the switch priority of a VLAN:

	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, the range is 1 to 4094. Do not enter leading zeros. •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 the Hello Time

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

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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.

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Beginning in privileged EXEC mode, follow these steps to configure the hello time of a VLAN:

	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, the range is 1 to 4094. Do not enter leading zeros. •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:

	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, the range is 1 to 4094. Do not enter leading zeros. •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, the range is 1 to 4094. Do not enter leading zeros. •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.

Configuring STP for Use in a Cascaded Stack

STP uses default values that can be reduced when configuring your switch in cascaded configurations. If a root switch is part of a cluster that is one switch from a cascaded stack, you can customize spanning tree to reconverge more quickly after a switch failure. Figure 14-4 shows switches in three cascaded stacks that use the GigaStack GBIC. Table 14-4 shows the default STP settings and those that are acceptable for these configurations.

Table 14-4 Default and Acceptable STP Parameter Settings (in seconds) 

STP Parameter	STP Default	Acceptable for Option 1	Acceptable for Option 2	Acceptable for Option 3
Hello Time	2	1	1	1
Max Age	20	6	10	6
Forwarding Delay	15	4	7	4

Figure 14-4 Gigabit Ethernet Stack

Displaying Spanning-Tree Status

To display the spanning-tree status, use one or more of the privileged EXEC commands in Table 14-5:

Table 14-5 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.

For information about other keywords for the show spanning-tree privileged EXEC command, refer to the Catalyst 3550 Multilayer Switch Command Reference for this release.