Table Of Contents

Configuring Spanning Tree

Understanding How Spanning Tree Works

Spanning Tree Overview

Election of the Root Switch

Determining the Spanning Tree Topology with BPDUs

Spanning Tree Timers

Creating the Spanning Tree Topology

STP Port States

Blocking State

Listening State

Learning State

Forwarding State

Disabled State

MAC Address Allocation

MAC Address Reduction

Spanning Tree and IEEE 802.1Q Trunks

Understanding How Spanning Tree for Token Ring Works

Default Spanning Tree Configuration

Configuring Spanning Tree

Enabling Spanning Tree

Enabling MAC Address Reduction

Configuring the Root Switch

Configuring a Secondary Root Switch

Configuring a Root Switch to Improve Convergence

Preventing Switches from Becoming the Root Switch

Configuring the Global Port Priority

Configuring the Port-VLAN Priority

Configuring Global Port Cost

Configuring Port-VLAN Cost

Configuring the Bridge Priority

Configuring the Hello Time

Configuring the Forward Delay Time

Configuring the Maximum Aging Time

Setting the STP Type for a TrBRF

Setting the Spanning Tree Port State

Specifying the STP Functional Address for a TrBRF

Disabling STP

Understanding How BPDU Skewing Works

Configuring Spanning Tree BPDU Skewing

Configuring Spanning Tree

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This chapter describes how to configure spanning tree on the Catalyst enterprise LAN switches.

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Note For information on configuring the PortFast, UplinkFast, and BackboneFast spanning tree enhancements, see Chapter 9, "Configuring Spanning Tree PortFast, UplinkFast, and BackboneFast."

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Note For complete syntax and usage information for the commands used in this chapter, refer to the Catalyst 5000 Family Command Reference.

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This chapter consists of these sections:

•Understanding How Spanning Tree Works

•Default Spanning Tree Configuration

•Configuring Spanning Tree

•Understanding How BPDU Skewing Works

•Configuring Spanning Tree BPDU Skewing

Understanding How Spanning Tree Works

These sections describe how spanning tree works:

•Spanning Tree Overview

•Election of the Root Switch

•Determining the Spanning Tree Topology with BPDUs

•Spanning Tree Timers

•Creating the Spanning Tree Topology

•STP Port States

•MAC Address Allocation

•Spanning Tree and IEEE 802.1Q Trunks

•Understanding How Spanning Tree for Token Ring Works

Spanning Tree Overview

The Spanning Tree Protocol (STP) uses a distributed algorithm that selects one bridge on a redundantly connected network as the root of a spanning-tree-connected active topology. STP assigns roles to each port depending on what the port's function is in the active topology. Port roles are as follows:

•Root—A unique 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. For more information, see the "Election of the Root Switch" section.

In Ethernet networks, only one active path may exist between any two stations. Multiple active paths between stations can cause loops in the network. When loops occur, some switches recognize stations on both sides of the switch. This situation causes the forwarding algorithm to malfunction allowing duplicate frames to be forwarded.

Spanning tree is a link management protocol that provides path redundancy while preventing undesirable loops in the network. For a Layer 2 Ethernet or Token Ring network to function properly, one active path must exist between 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.

The Catalyst enterprise LAN switches use the Spanning Tree Protocol on all Ethernet, Fast Ethernet, Gigabit Ethernet, and Token Ring port-based VLANs. By default, a single instance of STP runs on each configured VLAN (provided you do not manually disable STP). Depending on your hardware, you can enable and disable STP on a per-VLAN or a global basis.

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Note On a Catalyst 5000 family switch with Supervisor Engine II G or III G, or Supervisor Engine III with a NetFlow Feature Card (NFFC) or NFFC II, you cannot enable or disable STP on a per-VLAN basis. STP must be enabled or disabled for all VLANs on the switch using the set spantree {enable | disable} all command.

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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 network. Switches send and receive spanning tree frames at regular intervals. The switches do not forward these frames, but use the frames to construct a loop-free path.

Multiple active paths between stations cause loops in the network. If a loop exists in the network, hosts might receive duplicate messages. In addition, switches might learn host Media Access Control (MAC) addresses on multiple switch ports. 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 extended Layer 2 network. STP 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.

Election of the Root Switch

All switches in the Layer 2 network participating in spanning tree obtain information about other switches in the network through an exchange of data messages called Bridge Protocol Data Units (BPDUs).

BPDUs contain information about the transmitting switch and its ports, including switch and port MAC addresses, switch priority, port priority, and port cost. The STP uses this information to elect the root switch and root port for the switched network, as well as the root port and designated port for each switched segment.

The exchange of BPDUs results in the following actions:

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

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

•The removal of loops in the switched network by blocking switch ports connected to
redundant links

The switch with the highest bridge 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 Layer 2 network becomes the root switch.

The spanning tree 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 STP blocking mode.

Determining the Spanning Tree Topology with BPDUs

The active and active spanning tree topology of a switched network is determined by the following:

•The unique bridge ID (MAC address) associated with each switch

•The path cost to the root associated with each switch port

•The port identifier (MAC address) associated with each switch port

The switch sends configuration BPDUs to compute and communicate the spanning tree topology. Each configuration BPDU contains the following minimal information:

•The unique bridge ID of the switch that the transmitting switch believes to be the root switch

•The cost of the path to the root from the transmitting port

•The identifier of the transmitting port

When a switch transmits a BPDU frame, all switches connected to the LAN on which the frame is transmitted receive the BPDU. When a switch receives a BPDU, it does not forward the frame but instead uses the information in the frame to calculate a BPDU, and, if the topology changes, initiate a BPDU transmission.

A BPDU exchange results in the following:

•One switch is elected as the root switch.

•The shortest distance to the root switch is calculated for each switch.

•A designated switch is selected. This is the switch closest to the root switch through which frames will be forwarded to the root.

•A root port for each switch is selected. This is the port providing the best path from the switch to the root switch.

•Ports included in the spanning tree are selected.

Spanning Tree Timers

Table 8-1 describes the spanning tree timers that affect the entire spanning tree performance.

Table 8-1 Spanning Tree Timers

Variable	Description
Hello timer	Determines how often the switch broadcasts Hello messages to other switches.
Forward delay timer	Determines the amount of time a port will remain in the listening and learning states before entering the forwarding state.
Maximum age timer	Determines the amount of time protocol information received on a port is stored by the switch.

Creating the Spanning Tree Topology

In Figure 8-1, Switch A is elected as the root switch because the bridge priority of all the switches is set to the default (32768) and Switch A has the lowest MAC address. However, because of traffic patterns, the number of forwarding ports, or link types, Switch A might not be the best root switch. By increasing the priority (lowering the numerical priority value) of the best switch so that it becomes the root switch, you force an STP recalculation to form a new spanning tree topology with the best switch as the root.

Figure 8-1 Spanning Tree Topology

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

For example, assume that port 2 on Switch B is a fiber-optic link, and that port 1 on Switch B (an UTP link) is the root port. Network traffic might be more efficient over the high-speed fiber-optic link. By changing the spanning tree port priority or port cost for port 2 to a higher priority (lower numerical value) than port 1, port 2 becomes the root port.

STP Port 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 a switch port transitions directly from nonparticipation in the spanning tree topology to the forwarding state, it can create temporary data loops. Ports must wait for new topology information to propagate through the switched LAN before starting to forward frames. The ports must allow the frame lifetime to expire for frames that have been forwarded using the old topology.

Each port on a switch using STP exists in one of the following five states:

•Blocking

•Listening

•Learning

•Forwarding

•Disabled

A port moves through these five states as follows:

•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 8-2 illustrates how a port moves through the five states.

Figure 8-2 STP Port States

When you enable spanning tree, every switch in the network goes through the blocking state and the transitory states of listening and learning at power up. If properly configured, each port stabilizes to the forwarding or blocking state.

When the spanning tree algorithm places a port in the forwarding state, the following process occurs:

1. The port is put into the listening state while it waits for protocol information that suggests it should go to the blocking state.

2. The port waits for the expiration of the forward delay timer, moves the port to the learning state, and resets the forward delay timer.

3. In the learning state, the port continues to block frame forwarding as it learns station location information for the forwarding database.

4. The port waits for the expiration of the forward delay timer and then moves the port to the forwarding state, where both learning and forwarding are enabled.

Blocking State

A port in the blocking state does not participate in frame forwarding, as shown in Figure 8-3. After initialization, a BPDU is sent to each port in the switch. A switch initially assumes it is the root until it exchanges BPDUs with other switches. This exchange establishes which switch in the network is the root. If there is only one switch in the network, no exchange occurs, the forward delay timer expires, and the ports move to the listening state. A switch always enters the blocking state following switch initialization.

Figure 8-3 Port 2 in Blocking State

A port in the blocking state performs as follows:

•Discards frames received from the attached segment.

•Discards frames switched from another port for forwarding.

•Does not incorporate station location into its address database. (There is no learning on a blocking port, so there is no address database update.)

•Receives BPDUs and directs them to the system module.

•Does not transmit BPDUs received from the system module.

•Receives and responds to network management messages.

Listening State

The listening state is the first transitional state a port enters after the blocking state. The port enters this state when STP determines that the port should participate in frame forwarding. Figure 8-4 shows a port in the listening state.

Figure 8-4 Port 2 in Listening State

A port in the listening state performs as follows:

•Discards frames received from the attached segment.

•Discards frames switched from another port for forwarding.

•Does not incorporate station location into its address database. (There is no learning at this point, so there is no address database update.)

•Receives BPDUs and directs them to the system module.

•Processes BPDUs received from the system module.

•Receives and responds to network management messages.

Learning State

A port in the learning state prepares to participate in frame forwarding. The port enters the learning state from the listening state. Figure 8-5 shows a port in the learning state.

Figure 8-5 Port 2 in Learning State

A port in the learning state performs as follows:

•Discards frames received from the attached segment.

•Discards frames switched from another port for forwarding.

•Incorporates station location into its address database.

•Receives BPDUs and directs them to the system module.

•Receives, processes, and transmits BPDUs received from the system module.

•Receives and responds to network management messages.

Forwarding State

A port in the forwarding state forwards frames, as shown in Figure 8-6. The port enters the forwarding state from the learning state.

Figure 8-6 Port 2 in Forwarding State

A port in the forwarding state performs as follows:

•Forwards frames received from the attached segment.

•Forwards frames switched from another port for forwarding.

•Incorporates station location information into its address database.

•Receives BPDUs and directs them to the system module.

•Processes BPDUs received from the system module.

•Receives and responds to network management messages.

Disabled State

A port in the disabled state does not participate in frame forwarding or STP, as shown in Figure 8-7. A port in the disabled state is virtually nonoperational.

Figure 8-7 Port 2 in Disabled State

A disabled port performs as follows:

•Discards frames received from the attached segment.

•Discards frames switched from another port for forwarding.

•Does not incorporate station location into its address database. (There is no learning, so there is no address database update.)

•Receives BPDUs, but does not direct them to the system module.

•Does not receive BPDUs for transmission from the system module.

•Receives and responds to network management messages.

MAC Address Allocation

The supervisor engine has a pool of 1024 MAC addresses that are used as the bridge IDs for the VLAN spanning trees. You can use the show module command to view the MAC address range for the supervisor engine.

MAC addresses are allocated sequentially, with the first MAC address in the range assigned to VLAN 1, the second MAC address in the range assigned to VLAN 2, and so forth. The last MAC address in the range is assigned to the supervisor engine in-band (sc0) management interface.

For example, if the MAC address range for the supervisor engine is 00-e0-1e-9b-2e-00 to 00-e0-1e-9b-31-ff, the VLAN 1 bridge ID is 00-e0-1e-9b-2e-00, the VLAN 2 bridge ID is 00-e0-1e-9b-2e-01, the VLAN 3 bridge ID is 00-e0-1e-9b-2e-02, and so forth. The in-band (sc0) interface MAC address is 00-e0-1e-9b-31-ff.

MAC Address Reduction

The MAC address reduction feature is used on Catalyst 6000 family switches to enable extended-range VLAN identification. If you have a Catalyst 6000 switch in your network and you have MAC address reduction enabled on it, you should also enable it on all of your Catalyst 5000 family switches to avoid problems in the spanning tree topology. For detailed information on the MAC address reduction feature, refer to the Catalyst 6000 Software Configuration Guide.

Spanning Tree and IEEE 802.1Q Trunks

IEEE 802.1Q VLAN trunks impose some limitations on the spanning tree feature in a network. In a network of Cisco switches connected through 802.1Q trunks, the switches maintain one instance of spanning tree for each VLAN allowed on the trunks. However, non-Cisco 802.1Q switches maintain only one instance of spanning tree for all VLANs allowed on the trunks.

When you connect a Cisco switch to a non-Cisco device through an 802.1Q trunk, the Cisco switch combines the spanning tree instance of the 802.1Q native VLAN of the trunk with the spanning tree instance of the non-Cisco 802.1Q switch. However, all per-VLAN spanning tree 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.

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Note For more information on IEEE 802.1Q trunks, see Chapter 12, "Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports."

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Understanding How Spanning Tree for Token Ring Works

Token Ring runs spanning tree both at the Token Ring Concentrator Relay Function (TrCRF) level and the Token Ring Bridge Relay Function (TrBRF) level. The TrCRF spanning tree removes loops in the logical ring. The TrBRF spanning tree, like the Ethernet spanning tree, interacts with other switches to remove loops from the spanning tree topology.

The Catalyst 5000 family Token Ring modules support these STPs:

•IEEE 802.1D STP

•IBM STP

•Cisco STP

The Catalyst 5000 family switches use the IEEE 802.1D and IBM STPs on TrBRFs. The STP that runs on the TrCRF is either the Cisco STP or the IEEE STP, depending on the bridging mode you configured for the TrCRF with the set vlan command.

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Caution Some TrBRF STP and TrCRF bridge mode configurations are incompatible and can place the TrCRFs in a blocked state. For more information about these configurations, see the "Setting the Spanning Tree Port State" section.

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Default Spanning Tree Configuration

Table 8-2 shows the default spanning tree configuration.

Table 8-2 Spanning Tree Default Configuration 

Feature	Default Value
Enable state	Spanning tree enabled for all VLANs
Bridge priority	32768
Port priority	32 (global)
Port-VLAN priority	Same as port priority but configurable on a per-VLAN basis
Port cost	•Gigabit Ethernet: 4 •Fast Ethernet: 10 •FDDI/CDDI: 10 •Ethernet: 100 •Token Ring: 250
Port-VLAN cost	Same as port cost but configurable on a per-VLAN basis
Hello time	2 seconds
Forward delay time	12 seconds
Maximum aging time	20 seconds

Configuring Spanning Tree

These sections describe how to configure STP on any Ethernet, Fast Ethernet, Gigabit Ethernet, and Token Ring port-based VLANs:

•Enabling Spanning Tree

•Enabling MAC Address Reduction

•Configuring the Root Switch

•Configuring a Secondary Root Switch

•Preventing Switches from Becoming the Root Switch

•Configuring the Global Port Priority

•Configuring the Port-VLAN Priority

•Configuring Global Port Cost

•Configuring Port-VLAN Cost

•Configuring the Bridge Priority

•Configuring the Hello Time

•Configuring the Forward Delay Time

•Configuring the Maximum Aging Time

•Setting the STP Type for a TrBRF

•Setting the Spanning Tree Port State

•Specifying the STP Functional Address for a TrBRF

•Disabling STP

Enabling Spanning Tree

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Note Spanning tree is enabled by default on VLAN 1 and on all newly created VLANs.

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Depending on your hardware, you can enable spanning tree on a per-VLAN or a global basis. In either case, the switch maintains a separate instance of spanning tree for each VLAN (except on VLANs on which you disable spanning tree).

On Catalyst 5000 family switches with Supervisor Engine II G or III G, or with Supervisor Engine III or III F with NFFC or NFFC II, you must enable spanning tree globally for all VLANs using the all keyword.

If you do not specify the vlan value, VLAN 1 is assumed.

To enable spanning tree on a per-VLAN or global basis, perform this task in privileged mode:

	Task	Command
Step 1	Enable spanning tree on a per-VLAN or global basis.	set spantree enable [vlan] set spantree enable all
Step 2	Verify that spanning tree is enabled.	show spantree [vlan]

This example shows how to enable spanning tree globally for all VLANs on a Catalyst 5000 family switch with Supervisor Engine III G:

Console> (enable) set spantree enable all

Spantree enabled.

Console> (enable) 

This example shows how to enable spanning tree on VLAN 100:

Console> (enable) set spantree enable 100

Spantree 100 enabled.

Console> (enable)

Enabling MAC Address Reduction

To enable MAC address reduction, perform this task in privileged mode:

	Task	Command
	Enable MAC address reduction.	set spantree macreduction {enable | disable}

This example shows how to enable MAC address reduction:

Console> (enable) set spantree macreduction enable

MAC address reduction enabled

Console> (enable)

Configuring the Root Switch

The Catalyst enterprise LAN switches maintain a separate instance of the spanning tree for each active VLAN configured on the switch. A bridge ID (MAC address) and bridge priority are associated with each instance of spanning tree. The switch with the lowest bridge priority becomes the root switch for that instance of spanning tree.

When you configure a switch as the root, the spanning tree bridge priority is modified from the default value (32768) to a significantly lower value so that the switch becomes the root for the specified VLANs.

The switch checks the bridge priority of the current root switches for each VLAN. The bridge priority for the specified VLANs is set to 8192 if this value will cause the switch to become the root for the specified VLANs.

If any root switch for the specified VLANs has a bridge priority lower than 8192, the switch sets the bridge priority for the specified VLANs to 1 less than the lowest bridge priority.

For example, if all switches in the network have the bridge priority for VLANs 100 through 200 set to the default value of 32768, entering the set spantree root 100-200 command on a switch will set the bridge priority for VLANs 100 through 200 to 8192, causing the switch to become the root switch for those VLANs.

However, if the bridge priority for VLAN 150 on one of the other switches in the network is set to 4000, entering the set spantree root 100-200 command on another switch will set the bridge priority for VLANs 100 through 200 to 3999, causing the switch to become the root switch for those VLANs.

If reducing the bridge priority to 1 still does not make the switch the root switch for the specified VLANs, the system displays a message.

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

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Use the dia network_diameter keywords to specify the Layer 2 network diameter (that is, the maximum number of bridge hops between any two hosts in the Layer 2 network). When you specify the network diameter, the switch automatically picks an optimal Hello time, forward delay time, and maximum age time for a network of that diameter, which can significantly reduce the spanning tree convergence time. You can use the hello hello_time keywords to override the automatically calculated Hello time.

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Note We recommend that you avoid configuring the Hello time, forward delay time, and maximum age time manually after configuring the switch as the root.

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To configure a switch as the root switch, perform this task in privileged mode:

Task	Command
Configure a switch as the root switch.	set spantree root vlans [dia network_diameter] [hello hello_time]

This example shows how to configure the switch as the root switch for VLANs 1-10 with a network diameter of 4:

Console> (enable) set spantree root 1-10 dia 4

VLANs 1-10 bridge priority set to 8192

VLANs 1-10 bridge max aging time set to 14 seconds.

VLANs 1-10 bridge hello time set to 2 seconds.

VLANs 1-10 bridge forward delay set to 9 seconds.

Switch is now the root switch for active VLANs 1-6.

Console> (enable)

Configuring a Secondary Root Switch

When you configure a switch as the secondary root switch, the spanning tree bridge priority is modified from the default value (32768) to 16384 so that the switch will become the root for the specified VLANs if the primary root switch fails (assuming the other switches in the network use the default bridge priority of 32768).

You can run 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 configuring the primary root switch.

To configure a switch as the secondary root switch, perform this task in privileged mode:

Task	Command
Configure a switch as the secondary root switch.	set spantree root secondary vlans [dia network_diameter] [hello hello_time]

This example shows how to configure a switch as the secondary root switch for VLANs 22 and 24:

Console> (enable) set spantree root secondary 22,24 dia 5 hello 1

VLANs 22,24 bridge priority set to 16384.

VLANs 22,24 bridge max aging time set to 10 seconds.

VLANs 22,24 bridge hello time set to 1 second.

VLANs 22,24 bridge forward delay set to 7 seconds.

Console> (enable)

Configuring a Root Switch to Improve Convergence

You can configure the root switch to reduce the STP convergence time. To accomplish this task, you must reduce the values of the Hello Time, Forward Delay Timer, and Maximum Age Timer parameters. For information on configuring timers, see the following sections:

•Configuring the Hello Time

•Configuring the Forward Delay Time

•Configuring the Maximum Aging Time

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Note You can reduce the value of the timer parameters only if all of the links are LAN links of 10 megabits per second or faster. In this case, the network diameter can reach the maximum value of 7. With WAN connections, the parameters cannot be reduced.

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When a link failure occurs in a bridged network, the network reconfiguration does not occur immediately. Hello Time, Forward Delay timer, and Maximum Age timer reconfiguration takes 50 seconds with the default parameters (specified by IEEE 802.1D). The reconfiguration delay depends on the network diameter, which is the maximum number of bridges between any two end stations attachments.

To reduce convergence time, use the non-default parameter values that are permitted by the IEEE 802.1D standard. To set a reconvergence of 14 seconds, use the following non-default parameters:

Parameter	Time
Network diameter (dia)	2
Hello time	2 seconds
Forward Delay timer	4 seconds
Maximum Age timer	6 seconds

These parameters can be set on the Catalyst 5000 family switches without any modification to the switches.

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Note Switch ports can be set for improved convergence in PortFast mode. This setting only affects the transition from disable (link down) to enable (link up), which moves the port immediately to the forwarding state. If a port in the PortFast mode begins blocking, then it goes through listening and learning before reaching the forwarding state.

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To configure the spanning tree bridge to improve convergence time, perform these tasks in privileged mode:

	Task	Command
Step 1	Configure the Hello Time for a VLAN or MISTP instance.	set spantree hello interval [vlan] mistp-instance [instances]
Step 2	Verify the configuration.	show spantree [vlan | mistp-instance instances]
Step 3	Configure the Forward Delay time for a VLAN or MISTP instance.	set spantree fwddelay delay [vlan] mistp-instance [instances]
Step 4	Verify the configuration.	show spantree [mod/port] mistp-instance [instances] [active]
Step 5	Configure the Maximum Aging time for a VLAN or MISTP instance.	set spantree maxage agingtime [vlans] mistp-instance instances
Step 6	Verify the configuration.	show spantree [mod/port] mistp-instance [instances] [active]

This example shows how to configure the spanning tree Hello Time, Forward Delay timer, and Maximum Age timer to 2, 4, and 4 seconds respectively:

Console> (enable) set spantree hello 2 100

Spantree 100 hello time set to 7 seconds.

Console> (enable)

Console> (enable) set spantree fwddelay 4 100

Spantree 100 forward delay set to 21 seconds.

Console> (enable)

Console> (enable) set spantree maxage 6 100

Spantree 100 max aging time set to 36 seconds.

Console> (enable)

Console> (enable) set spantree root 1-10 dia 4

VLANs 1-10 bridge priority set to 8192

VLANs 1-10 bridge max aging time set to 14 seconds.

VLANs 1-10 bridge hello time set to 2 seconds.

VLANs 1-10 bridge forward delay set to 9 seconds.

Switch is now the root switch for active VLANs 1-6.

Console> (enable) 

Preventing Switches from Becoming the Root Switch

You can prevent switches from becoming the root switch by using the root guard feature. The root guard feature forces a port to become a designated port so that no switch on the other end of the link can become a root switch.

When you enable root guard on a per-port basis, it automatically applies to all of the active VLANs to which that port belongs. When you disable root guard, it is disabled for the specified port. If a port goes into the root-inconsistent state, it automatically goes into the listening state.

To prevent switches from becoming root, perform this task in privileged mode.

	Task	Command
Step 1	Enable root guard on a port.	set spantree guard {root | none} {mod/port}
Step 2	Verify that root guard is enabled.	show spantree guard {root | none}{mod/port | vlan}

Configuring the Global Port Priority

You can change the global port priority of switch ports. The port with the lowest priority value forwards frames for all VLANs. The possible priority value range is 0 through 63. If all ports have the same priority value, the port with the lowest port number forwards frames.

To change the global port priority for a port, perform this task in privileged mode:

	Task	Command
Step 1	Change the global port priority for a switch port.	set spantree portpri mod/port priority
Step 2	Verify the port priority setting.	show spantree [mod/port]

This example shows how to change the global port priority for a port and verify the configuration:

Console> (enable) set spantree portpri 1/2 20

Bridge port 1/2 port priority set to 20.

Console> (enable) show spantree 1/2

Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method

--------- ----  -------------  -----  --------  ----------  ------------

 1/2      1     blocking          19        20   disabled              

 1/2      100   forwarding        19        20   disabled              

 1/2      521   blocking          19        20   disabled              

 1/2      522   blocking          19        20   disabled              

 1/2      523   blocking          19        20   disabled              

 1/2      524   blocking          19        20   disabled              

 1/2      1003  not-connected     19        20   disabled              

 1/2      1005  not-connected     19         4   disabled              

Console> (enable)

Configuring the Port-VLAN Priority

You can set the port priority for a port on a per-VLAN basis. The port with the lowest priority value for a specific VLAN forwards frames for that VLAN. The possible priority value range is 0 through 63. If all ports have the same priority value for a particular VLAN, the port with the lowest port number forwards frames for that VLAN. If you do not specify the vlan value, VLAN 1 is assumed as the value.

To change the port-VLAN priority for a port, perform this task in privileged mode:

	Task	Command
Step 1	Change the port-VLAN priority for a VLAN on a switch port.	set spantree portvlanpri mod/port priority [vlans]
Step 2	Verify the port-VLAN priority setting.	show spantree [mod/port]

This example shows how to change the port-VLAN priority on a port and verify the configuration:

Console> (enable) set spantree portvlanpri 1/2 1 100

Port 1/2 vlans 1-99,101-1004 using portpri 32.

Port 1/2 vlans 100 using portpri 1.

Port 1/2 vlans 1005 using portpri 4.

Console> (enable) show spantree 1/2

Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method

--------- ----  -------------  -----  --------  ----------  ------------

 1/2      1     blocking          19        32   disabled              

 1/2      100   forwarding        19         1   disabled              

 1/2      521   blocking          19        32   disabled              

 1/2      522   blocking          19        32   disabled              

 1/2      523   blocking          19        32   disabled              

 1/2      524   blocking          19        32   disabled              

 1/2      1003  not-connected     19        32   disabled              

 1/2      1005  not-connected     19         4   disabled              

Console> (enable) 

Configuring Global Port Cost

You can change the global port cost of switch ports. Ports with lower port costs are more likely to be used to forward frames for all VLANs. Assign lower numbers to ports attached to faster media (such as full-duplex Fast Ethernet) and higher numbers to ports attached to slower media (such as half-duplex Ethernet). The possible range of the cost value is 1 to 65535.

To change the global port cost for a port, perform this task in privileged mode:

	Task	Command
Step 1	Change the global port cost for a switch port.	set spantree portcost mod/port cost
Step 2	Verify the port cost setting.	show spantree [mod/port]

This example shows how to change the global port cost on a port and verify the configuration:

Console> (enable) set spantree portcost 1/2 10

Spantree port 1/2 path cost set to 10.

Console> (enable) show spantree 1/2

Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method

--------- ----  -------------  -----  --------  ----------  ------------

 1/2      1     forwarding        10        20   disabled              

 1/2      100   forwarding        10        20   disabled              

 1/2      521   forwarding        10        20   disabled              

 1/2      522   forwarding        10        20   disabled              

 1/2      523   forwarding        10        20   disabled              

 1/2      524   forwarding        10        20   disabled              

 1/2      1003  not-connected     10        20   disabled              

 1/2      1005  not-connected     10         4   disabled              

Console> (enable)

Configuring Port-VLAN Cost

You can change the port cost for a port on a per-VLAN basis. Ports with lower port-VLAN costs are more likely to be used to forward frames for those VLANs. You should assign lower numbers to ports attached to faster media (such as full-duplex Fast Ethernet) and higher numbers to ports attached to slower media (such as half-duplex Ethernet). The possible range of the cost value is 1 to 65535. If you do not specify the vlan value, VLAN 1 is assumed as the value.

To change the port-VLAN cost for a port, perform this task in privileged mode:

	Task	Command
Step 1	Change the port-VLAN cost for a VLAN on a switch port.	set spantree portvlancost mod/port cost cost [vlans]
Step 2	Verify the port-VLAN cost setting.	show spantree [mod/port]

This example shows how to change the port-VLAN cost on a port and verify the configuration:

Console> (enable) set spantree portvlancost 1/2 cost 10 100

Port 1/2 VLANs 1-99,101-1005 have path cost 19.

Port 1/2 VLANs 100 have path cost 10.

Console> (enable) show spantree 1/2

Port      Vlan  Port-State     Cost   Priority  Fast-Start  Group-method

--------- ----  -------------  -----  --------  ----------  ------------

 1/2      1     blocking          19        20   disabled              

 1/2      100   forwarding        10        20   disabled              

 1/2      521   blocking          19        20   disabled              

 1/2      522   blocking          19        20   disabled              

 1/2      523   blocking          19        20   disabled              

 1/2      524   blocking          19        20   disabled              

 1/2      1003  not-connected     19        20   disabled              

 1/2      1005  not-connected     19         4   disabled              

Console> (enable)

Configuring the Bridge Priority

Use the set spantree priority command to manually change the spanning tree bridge priority for a VLAN. The possible range of the bridge_priority value is 0 to 65535. If you do not specify the vlan value, VLAN 1 is assumed as the value.

---

Note Be careful when using this command. For most situations, we recommend that you use the set spantree root and set spantree root secondary commands to modify the bridge priority and related parameters.

---

To configure the spanning tree bridge priority for a VLAN, perform this task in privileged mode:

	Task	Command
Step 1	Set the bridge priority for a VLAN.	set spantree priority bridge_priority [vlan]
Step 2	Verify the configuration.	show spantree [vlan]

This example shows how to change the spanning tree bridge priority for VLAN 100 to 8192:

Console> (enable) set spantree priority 8192 100

Spantree 100 bridge priority set to 8192.

Console> (enable) 

Configuring the Hello Time

Use the set spantree hello command to manually change the spanning tree Hello time for a VLAN. The possible range of the interval value is 1 to 10 seconds. If you do not specify the vlan, VLAN 1 is assumed as the value.

---

Note Be careful when using this command. For most situations, we recommend that you use the set spantree root and set spantree root secondary commands to modify the Hello time and related parameters.

---

To configure the spanning tree bridge Hello time for a VLAN, perform this task in privileged mode:

	Task	Command
Step 1	Set the Hello time for a VLAN.	set spantree hello interval [vlan]
Step 2	Verify the configuration.	show spantree [vlan]

This example shows how to change the spanning tree Hello time for VLAN 100 to 7 seconds:

Console> (enable) set spantree hello 7 100

Spantree 100 hello time set to 7 seconds.

Console> (enable)

Configuring the Forward Delay Time

Use the set spantree fwddelay command to manually change the spanning tree forward delay time for a VLAN. The possible range of the delay value is 4 to 30 seconds. If you do not specify the vlan value, VLAN 1 is assumed as the value.

---

Note Be careful when using this command. For most situations, we recommend that you use the set spantree root and set spantree root secondary commands to modify the forward delay time and related parameters.

---

To configure the spanning tree forward delay time for a VLAN, perform this task in privileged mode:

	Task	Command
Step 1	Set the forward delay time for a VLAN.	set spantree fwddelay delay_time [vlan]
Step 2	Verify the configuration.	show spantree [vlan]

This example shows how to change the spanning tree forward delay time for VLAN 100 to 21 seconds:

Console> (enable) set spantree fwddelay 21 100

Spantree 100 forward delay set to 21 seconds.

Console> (enable)

Configuring the Maximum Aging Time

Use the set spantree maxage command to manually change the spanning tree maximum aging time for a VLAN. The possible range of the agingtime value is 6 to 40 seconds. If you do not specify the vlan value, VLAN 1 is assumed as the value.

---

Note Be careful when using this command. For most situations, we recommend that you use the set spantree root and set spantree root secondary commands to modify the maximum aging time and related parameters.

---

To configure the spanning tree maximum aging time for a VLAN, perform this task in privileged mode:

	Task	Command
Step 1	Set the maximum aging time for a VLAN.	set spantree maxage agingtime [vlan]
Step 2	Verify the configuration.	show spantree [vlan]

This example shows how to change the spanning tree maximum aging time for VLAN 100 to 36 seconds:

Console> (enable) set spantree maxage 36 100

Spantree 100 max aging time set to 36 seconds.

Console> (enable)

Setting the STP Type for a TrBRF

You can configure the STP type to be used by a TrBRF. The following STP and bridge mode configurations are incompatible and can place logical ports in a blocked state:

•TrBRF is running the IBM STP and the TrCRF is in SRT mode.

•TrBRF is running the IEEE STP and the TrCRF is in SRB mode.

For more information, see the "Setting the Spanning Tree Port State" section.

To specify the STP type for a TrBRF, perform this task in privileged mode:

Task