To assign an Ethernet port to an EtherChannel group, or to enable an EtherChannel mode, or both, use the channel-group command in interface configuration mode. To remove an Ethernet port from an EtherChannel group, use the no form of this command.

Command Default

No channel groups are assigned.

No mode is configured.

Command Modes

Interface configuration

Usage Guidelines

The IP Lite feature set supports up to 48 EtherChannels.

For Layer 2 EtherChannels, the channel-group command automatically creates the port-channel interface when the channel group gets its first physical port. You do not have to use the interface port-channel command in global configuration mode to manually create a port-channel interface. If you create the port-channel interface first, the channel-group-number can be the same as the port-channel-number, or you can use a new number. If you use a new number, the channel-group command dynamically creates a new port channel.

Although it is not necessary to disable the IP address that is assigned to a physical port that is part of a channel group, we strongly recommend that you do so.

You create Layer 3 port channels by using the interface port-channel command followed by the no switchport interface configuration command. Manually configure the port-channel logical interface before putting the interface into the channel group.

After you configure an EtherChannel, configuration changes that you make on the port-channel interface apply to all the physical ports assigned to the port-channel interface. Configuration changes applied to the physical port affect only the port where you apply the configuration. To change the parameters of all ports in an EtherChannel, apply configuration commands to the port-channel interface, for example, spanning-tree commands or commands to configure a Layer 2 EtherChannel as a trunk.

Active mode places a port into a negotiating state in which the port initiates negotiations with other ports by sending LACP packets. A channel is formed with another port group in either the active or passive mode.

Auto mode places a port into a passive negotiating state in which the port responds to PAgP packets it receives but does not start PAgP packet negotiation. A channel is formed only with another port group in desirable mode. When auto is enabled, silent operation is the default.

Desirable mode places a port into an active negotiating state in which the port starts negotiations with other ports by sending PAgP packets. An EtherChannel is formed with another port group that is in the desirable or auto mode. When desirable is enabled, silent operation is the default.

If you do not specify non-silent with the auto or desirable mode, silent is assumed. The silent mode is used when the switch is connected to a device that is not PAgP-capable and rarely, if ever, sends packets. An example of a silent partner is a file server or a packet analyzer that is not generating traffic. In this case, running PAgP on a physical port prevents that port from ever becoming operational. However, it allows PAgP to operate, to attach the port to a channel group, and to use the port for transmission. Both ends of the link cannot be set to silent.

In on mode, a usable EtherChannel exists only when both connected port groups are in the on mode.

Caution

Use care when using the on mode. This is a manual configuration, and ports on both ends of the EtherChannel must have the same configuration. If the group is misconfigured, packet loss or spanning-tree loops can occur.

Passive mode places a port into a negotiating state in which the port responds to received LACP packets but does not initiate LACP packet negotiation. A channel is formed only with another port group in active mode.

Do not configure an EtherChannel in both the PAgP and LACP modes. EtherChannel groups running PAgP and LACP can coexist on the same switch or on different switches in the stack (but not in a cross-stack configuration). Individual EtherChannel groups can run either PAgP or LACP, but they cannot interoperate.

If you set the protocol by using the channel-protocol interface configuration command, the setting is not overridden by the channel-group interface configuration command.

Do not configure a port that is an active or a not-yet-active member of an EtherChannel as an IEEE 802.1x port. If you try to enable IEEE 802.1x authentication on an EtherChannel port, an error message appears, and IEEE 802.1x authentication is not enabled.

Do not configure a secure port as part of an EtherChannel or configure an EtherChannel port as a secure port.

For a complete list of configuration guidelines, see the “Configuring EtherChannels” chapter in the software configuration guide for this release.

Caution

Do not enable Layer 3 addresses on the physical EtherChannel ports. Do not assign bridge groups on the physical EtherChannel ports because it creates loops.

To restrict the protocol used on a port to manage channeling, use the channel-protocol command in interface configuration mode. To return to the default setting, use the no form of this command.

Command Default

No protocol is assigned to the EtherChannel.

Command Modes

Interface configuration

Usage Guidelines

Use the channel-protocol command only to restrict a channel to LACP or PAgP. If you set the protocol by using the channel-protocol command, the setting is not overridden by the channel-group interface configuration command.

You must use the channel-group interface configuration command to configure the EtherChannel parameters. The channel-group command also can set the mode for the EtherChannel.

You cannot enable both the PAgP and LACP modes on an EtherChannel group.

PAgP and LACP are not compatible; both ends of a channel must use the same protocol.

You cannot configure PAgP on cross-stack configurations.

To clear Link Aggregation Control Protocol (LACP) channel-group counters, use the clear lacp command in privileged EXEC mode.

Command Default

None

Command Modes

Privileged EXEC

Usage Guidelines

You can clear all counters by using the clear lacp counters command, or you can clear only the counters for the specified channel group by using the clear lacp channel-group-number counters command.

To clear the Port Aggregation Protocol (PAgP) channel-group information, use the clear pagp command in privileged EXEC mode.

Command Default

None

Command Modes

Privileged EXEC

Usage Guidelines

You can clear all counters by using the clear pagp counters command, or you can clear only the counters for the specified channel group by using the clear pagp channel-group-number counters command.

To clear the spanning-tree counters, use the clear spanning-tree counters command in privileged EXEC mode.

Command Default

None

Command Modes

Privileged EXEC

Usage Guidelines

If the interface-id value is not specified, spanning-tree counters are cleared for all interfaces.

To restart the protocol migration process and force renegotiation with neighboring switches on the interface, use the clear spanning-tree detected-protocols command in privileged EXEC mode.

Command Default

None

Command Modes

Privileged EXEC

Usage Guidelines

A switch running the rapid per-VLAN spanning-tree plus (rapid-PVST+) protocol or the Multiple Spanning Tree Protocol (MSTP) supports a built-in protocol migration method that enables it to interoperate with legacy IEEE 802.1D switches. If a rapid-PVST+ or an MSTP switch receives a legacy IEEE 802.1D configuration bridge protocol data unit (BPDU) with the protocol version set to 0, the switch sends only IEEE 802.1D BPDUs on that port. A multiple spanning-tree (MST) switch can also detect that a port is at the boundary of a region when it receives a legacy BPDU, an MST BPDU (Version 3) associated with a different region, or a rapid spanning-tree (RST) BPDU (Version 2).

The switch does not automatically revert to the rapid-PVST+ or the MSTP mode if it no longer receives IEEE 802.1D BPDUs because it cannot learn whether the legacy switch has been removed from the link unless the legacy switch is the designated switch. Use the clear spanning-tree detected-protocols command in this situation.

To enable debugging of EtherChannels, use the debug etherchannel command in privileged EXEC mode. To disable debugging, use the no form of the command.

Command Default

Debugging is disabled.

Command Modes

Privileged EXEC

Usage Guidelines

The undebug etherchannel command is the same as the no debug etherchannel command.

Note	Although the linecard keyword is displayed in the command-line help, it is not supported.

When you enable debugging on a stack, it is enabled only on the stack master. To enable debugging on a stack member , start a session from the stack master by using the session switch-number command in privileged EXEC mode. Enter the debug command at the command-line prompt of the stack member.

To enable debugging on a stack member without first starting a session on the stack master, use the remote command switch-number LINE command in privileged EXEC mode.

To enable debugging of Link Aggregation Control Protocol (LACP) activity, use the debug lacp command in privileged EXEC mode. To disable LACP debugging, use the no form of this command.

Command Default

Debugging is disabled.

Command Modes

Privileged EXEC

Usage Guidelines

The undebug etherchannel command is the same as the no debug etherchannel command.

When you enable debugging on a stack, it is enabled only on the stack master. To enable debugging on a stack member , start a session from the stack master by using the session switch-number command in privileged EXEC mode. Enter the debug command at the command-line prompt of the stack member.

To enable debugging on a stack member without first starting a session on the stack master, use the remote command switch-number LINE command in privileged EXEC mode.

To enable debugging of Port Aggregation Protocol (PAgP) activity, use the debug pagp command in privileged EXEC mode. To disable PAgP debugging, use the no form of this command.

Command Default

Debugging is disabled.

Command Modes

Privileged EXEC

Usage Guidelines

The undebug pagp command is the same as the no debug pagp command.

When you enable debugging on a stack, it is enabled only on the stack master. To enable debugging on a stack member , start a session from the stack master by using the session switch-number command in privileged EXEC mode. Enter the debug command at the command-line prompt of the stack member.

To enable debugging on a stack member without first starting a session on the stack master, use the remote command switch-number LINE command in privileged EXEC mode.

To enable debugging of platform-dependent EtherChannel events, use the debug platform etherchannel command in EXEC mode. To disable debugging, use the no form of this command.

Command Default

Debugging is disabled.

Command Modes

User EXEC

Privileged EXEC

Usage Guidelines

The undebug platform etherchannel command is the same as the no debug platform etherchannel command.

When you enable debugging on a stack, it is enabled only on the stack master. To enable debugging on a stack member , start a session from the stack master by using the session switch-number command in privileged EXEC mode. Enter the debug command at the command-line prompt of the stack member.

To enable debugging on a stack member without first starting a session on the stack master, use the remote command switch-number LINE command in privileged EXEC mode.

To enable debugging of the platform-dependent port manager software module, use the debug platform pm command in privileged EXEC mode. To disable debugging, use the no form of this command.

Syntax Description

Command Default

Debugging is disabled

Command Modes

Privileged EXEC

Usage Guidelines

The undebug platform pm command is the same as the no debug platform pm command.

When you enable debugging on a stack, it is enabled only on the stack master. To enable debugging on a stack member , start a session from the stack master by using the session switch-number command in privileged EXEC mode. Enter the debug command at the command-line prompt of the stack member.

To enable debugging on a stack member without first starting a session on the stack master, use the remote command switch-number LINE command in privileged EXEC mode.

To enable debugging of spanning-tree activities, use the debug spanning-tree command in EXEC mode. To disable debugging, use the no form of this command.

Syntax Description

Command Default

Debugging is disabled.

Command Modes

Privileged EXEC

Usage Guidelines

The undebug spanning-tree command is the same as the no debug spanning-tree command.

When you enable debugging on a stack, it is enabled only on the stack master. To enable debugging on a stack member , start a session from the stack master by using the session switch-number command in privileged EXEC mode. Enter the debug command at the command-line prompt of the stack member.

To enable debugging on a stack member without first starting a session on the stack master, use the remote command switch-number LINE command in privileged EXEC mode.

To enable debugging of the platform-dependent UniDirectional Link Detection (UDLD) software, use the debug platform udld command in privileged EXEC mode. To disable debugging, use the no form of this command.

Command Default

Debugging is disabled.

Command Modes

Privileged EXEC

Usage Guidelines

The undebug platform udld command is the same as the no debug platform udld command.

When you enable debugging on a stack, it is enabled only on the stack master. To enable debugging on a stack member , start a session from the stack master by using the session switch-number command in privileged EXEC mode. Enter the debug command at the command-line prompt of the stack member.

To enable debugging on a stack member without first starting a session on the stack master, use the remote command switch-number LINE command in privileged EXEC mode.

To access or create a port channel, use the interface port-channel command in global configuration mode. Use the no form of this command to remove the port channel.

Command Default

No port channel logical interfaces are defined.

Command Modes

Global configuration

Usage Guidelines

For Layer 2 EtherChannels, you do not have to create a port-channel interface before assigning physical ports to a channel group. Instead, you can use the channel-group interface configuration command, which automatically creates the port-channel interface when the channel group obtains its first physical port. If you create the port-channel interface first, the channel-group-number can be the same as the port-channel-number, or you can use a new number. If you use a new number, the channel-group command dynamically creates a new port channel.

You create Layer 3 port channels by using the interface port-channel command followed by the no switchport interface configuration command. You should manually configure the port-channel logical interface before putting the interface into the channel group.

Only one port channel in a channel group is allowed.

Caution

When using a port-channel interface as a routed port, do not assign Layer 3 addresses on the physical ports that are assigned to the channel group.

Caution

Do not assign bridge groups on the physical ports in a channel group used as a Layer 3 port channel interface because it creates loops. You must also disable spanning tree.

Follow these guidelines when you use the interface port-channel command:

• If you want to use the Cisco Discovery Protocol (CDP), you must configure it on the physical port and not on the port channel interface.
• Do not configure a port that is an active member of an EtherChannel as an IEEE 802.1x port. If IEEE 802.1x is enabled on a not-yet active port of an EtherChannel, the port does not join the EtherChannel.

For a complete list of configuration guidelines, see the “Configuring EtherChannels” chapter in the software configuration guide for this release.

To configure the port priority for the Link Aggregation Control Protocol (LACP), use the lacp port-priority command in interface configuration mode. To return to the default setting, use the no form of this command.

Command Default

The default is 32768.

Command Modes

Interface configuration

Usage Guidelines

The lacp port-priority interface configuration command determines which ports are bundled and which ports are put in hot-standby mode when there are more than eight ports in an LACP channel group.

An LACP channel group can have up to 16 Ethernet ports of the same type. Up to eight ports can be active, and up to eight ports can be in standby mode.

In port-priority comparisons, a numerically lower value has a higher priority: When there are more than eight ports in an LACP channel group, the eight ports with the numerically lowest values (highest priority values) for LACP port priority are bundled into the channel group, and the lower-priority ports are put in hot-standby mode. If two or more ports have the same LACP port priority (for example, they are configured with the default setting of 65535), then an internal value for the port number determines the priority.

Note	The LACP port priorities are only effective if the ports are on the switch that controls the LACP link. See the lacp system-priority global configuration command for determining which switch controls the link.

Use the show lacp internal privileged EXEC command to display LACP port priorities and internal port number values.

For information about configuring LACP on physical ports, see the configuration guide for this release.

To configure the system priority for the Link Aggregation Control Protocol (LACP), use the lacp system-priority command in global configuration mode on the switch. To return to the default setting, use the no form of this command.

Command Default

The default is 32768.

Command Modes

Global configuration

Usage Guidelines

The lacp system-priority command determines which switch in an LACP link controls port priorities.

An LACP channel group can have up to 16 Ethernet ports of the same type. Up to eight ports can be active, and up to eight ports can be in standby mode. When there are more than eight ports in an LACP channel group, the switch on the controlling end of the link uses port priorities to determine which ports are bundled into the channel and which ports are put in hot-standby mode. Port priorities on the other switch (the noncontrolling end of the link) are ignored.

In priority comparisons, numerically lower values have a higher priority. Therefore, the system with the numerically lower value (higher priority value) for LACP system priority becomes the controlling system. If both switches have the same LACP system priority (for example, they are both configured with the default setting of 32768), the LACP system ID (the switch MAC address) determines which switch is in control.

The lacp system-priority command applies to all LACP EtherChannels on the switch.

Use the show etherchannel summary privileged EXEC command to see which ports are in the hot-standby mode (denoted with an H port-state flag in the output display).

To configure an interface as a member of a link-state group, use the link state group command in interface configuration mode. Use the no form of this command to remove an interface from a link-state group.

Command Default

No link-state group is configured.

Command Modes

Interface configuration

Usage Guidelines

Add upstream interfaces to the link-state group before adding downstream interfaces, otherwise, the downstream interfaces move into error-disable mode. These are the limitations:

• An interface can be an upstream interface or a downstream interface.
• An interface can belong to only one link-state group.
• Only two link-state groups can be configured on a switch.

To enable a link-state group, use the link state track command in global configuration mode. Use the no form of this command to disable a link-state group.

Command Default

Link-state tracking is disabled.

Command Modes

Global configuration

Usage Guidelines

Use the link state group command to create and configure the link-state group. You then can use this command to enable the link-state group.

To learn the source address of incoming packets received from an EtherChannel port, use the pagp learn-method command in interface configuration mode. To return to the default setting, use the no form of this command.

Command Default

The default is aggregation-port (logical port channel).

Command Modes

Interface configuration

Usage Guidelines

The learn method must be configured the same at both ends of the link.

The switch supports address learning only on aggregate ports even though the physical-port keyword is provided in the command-line interface (CLI). The pagp learn-method and the pagp port-priority interface configuration commands have no effect on the switch hardware, but they are required for PAgP interoperability with devices that only support address learning by physical ports.

When the link partner to the switch is a physical learner, we recommend that you configure the switch as a physical-port learner by using the pagp learn-method physical-port interface configuration command. We also recommend that you set the load-distribution method based on the source MAC address by using the port-channel load-balance src-mac global configuration command. Use the pagp learn-method interface configuration command only in this situation.

To select a port over which all Port Aggregation Protocol (PAgP) traffic through the EtherChannel is sent, use the pagp port-priority command in interface configuration mode. If all unused ports in the EtherChannel are in hot-standby mode, they can be placed into operation if the currently selected port and link fails. To return to the default setting, use the no form of this command.

Command Default

The default is 128.

Command Modes

Interface configuration

Usage Guidelines

The physical port with the highest priority that is operational and has membership in the same EtherChannel is the one selected for PAgP transmission.

The switch supports address learning only on aggregate ports even though the physical-port keyword is provided in the command-line interface (CLI). The pagp learn-method and the pagp port-priority interface configuration commands have no effect on the switch hardware, but they are required for PAgP interoperability with devices that only support address learning by physical ports, such as the Catalyst 1900 switch.

When the link partner to the switch is a physical learner, we recommend that you configure the switch as a physical-port learner by using the pagp learn-method physical-port interface configuration command. We also recommend that you set the load-distribution method based on the source MAC address by using the port-channel load-balance src-mac global configuration command. Use the pagp learn-method interface configuration command only in this situation.

To set the PAgP timer expiration, use the pagp timer command in interface configuration mode. To return to the default setting, use the no form of this command.

Command Default

None

Command Modes

Interface configuration

Usage Guidelines

This command is available for all interfaces configured as part of a PAgP port channel.

To set the load-distribution method among the ports in the EtherChannel, use the port-channel load-balance command in global configuration mode. To reset the load-balancing function to the default setting, use the no form of this command.

Command Default

The default is src-mac.

Command Modes

Global configuration

Usage Guidelines

You can verify your setting by entering the show running-config privileged EXEC command or the show etherchannel load-balance privileged EXEC command.

To display EtherChannel information for a channel, use the show etherchannel command in user EXEC mode.

Command Default

None

Command Modes

User EXEC

Usage Guidelines

If you do not specify a channel group number, all channel groups are displayed.

In the output, the passive port list field is displayed only for Layer 3 port channels. This field means that the physical port, which is still not up, is configured to be in the channel group (and indirectly is in the only port channel in the channel group).

To display Link Aggregation Control Protocol (LACP) channel-group information, use the show lacp command in user EXEC mode.

Command Default

None

Command Modes

User EXEC

Usage Guidelines

You can enter any show lacp command to display the active channel-group information. To display specific channel information, enter the show lacp command with a channel-group number.

If you do not specify a channel group, information for all channel groups appears.

You can enter the channel-group-number to specify a channel group for all keywords except sys-id.

To display link-state group information, use the show link state group command in privileged EXEC mode.

Command Default

None

Command Modes

Privileged EXEC

Usage Guidelines

To display information about all link-state groups, enter this command without keywords. To display information about a specific link-state group enter the link-state group number.

The output for the show link state group detail displays information for only those link-state groups that have link-state tracking enabled or that have upstream or downstream interfaces configured. If the group does not have a configuration, the group is not shown as enabled or disabled.

To display Port Aggregation Protocol (PAgP) channel-group information, use the show pagp command in EXEC mode.

Command Default

None

Command Modes

User EXEC

Privileged EXEC

Usage Guidelines

You can enter any show pagp command to display the active channel-group information. To display the nonactive information, enter the show pagp command with a channel-group number.

To display platform-dependent backup information used in a Flex Links configuration, use the show platform backup interface privileged EXEC command.

Command Modes

Privileged EXEC

Usage Guidelines

Use this command only when you are working directly with a technical support representative while troubleshooting a problem.

Do not use this command unless a technical support representative asks you to do so.

To display platform-dependent EtherChannel information, use the show platform etherchannel command in privileged EXEC mode.

Syntax Description

Command Default

None

Command Modes

Privileged EXEC

Usage Guidelines

Use this command only when you are working directly with a technical support representative while troubleshooting a problem.

Do not use this command unless a technical support representative asks you to do so.

To display platform-dependent port manager information, use the show platform pm command in privileged EXEC mode.

Command Default

None

Command Modes

Privileged EXEC

Usage Guidelines

Use this command only when you are working directly with your technical support representative while troubleshooting a problem.

Do not use this command unless your technical support representative asks you to do so.

To display platform-dependent spanning-tree information, use the show platform spanning-tree privileged EXEC command.

Command Modes

Privileged EXEC

Usage Guidelines

Use this command only when you are working directly with your technical support representative while troubleshooting a problem.

Do not use this command unless your technical support representative asks you to do so.

To display spanning-tree information for the specified spanning-tree instances, use the show spanning-tree command in privileged EXEC mode or user EXEC mode.

Syntax Description

Command Modes

User EXEC

Privileged EXEC

Usage Guidelines

If you do not specify a vlan-id value when you use the vlan keyword, the command applies to spanning-tree instances for all VLANs.

To display UniDirectional Link Detection (UDLD) administrative and operational status for all ports or the specified port, use the show udld command in user EXEC mode.

Command Default

None

Command Modes

User EXEC

Usage Guidelines

If you do not enter an interface ID, administrative and operational UDLD status for all interfaces appear.

To enable BackboneFast to allow a blocked port on a switch to change immediately to a listening mode, use the spanning-tree backbonefast command in global configuration mode. To return to the default setting, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

BackboneFast is disabled.

Command Modes

Global configuration

Command History

Usage Guidelines

Enable BackboneFast so that the switch detects indirect link failures and starts the spanning-tree reconfiguration sooner than it would under normal spanning-tree rules.

You can configure BackboneFast for rapid PVST+ or for multiple spanning-tree (MST) mode; however, the feature remains disabled until you change the spanning-tree mode to PVST+.

Use the show spanning-tree privileged EXEC command to verify your settings.

To enable bridge protocol data unit (BPDU) filtering on the interface, use the spanning-tree bpdufilter command in interface configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

The setting that is already configured when you enter the spanning-tree portfast bpdufilter default command.

Command Modes

Interface configuration

Command History

Usage Guidelines

This command has three states:

• spanning-tree bpdufilter enable —Unconditionally enables BPDU filtering on the interface.
• spanning-tree bpdufilter disable —Unconditionally disables BPDU filtering on the interface.
• no spanning-tree bpdufilter —Enables BPDU filtering on the interface if the interface is in the operational PortFast state and if you configure the spanning-tree portfast bpdufilter default command.

Caution

Be careful when you enter the spanning-tree bpdufilter enable command. Enabling BPDU filtering on an interface is similar to disabling the spanning tree for this interface. If you do not use this command correctly, you might create bridging loops.

You can enable BPDU filtering when the switch is operating in the per-VLAN spanning-tree plus (PVST+) mode, the rapid-PVST mode, or the multiple spanning-tree (MST) mode.

You can globally enable BPDU filtering on all Port Fast-enabled interfaces with the spanning-tree portfast bpdufilter default command.

The spanning-tree bpdufilter enable command overrides the PortFast configuration.

To enable bridge protocol data unit (BPDU) guard on the interface, use the spanning-tree bpduguard command in interface configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

The setting that is already configured when you enter the spanning-tree portfast bpduguard default command.

Command Modes

Interface configuration

Command History

Usage Guidelines

Use the BPDU guard feature in a service-provider environment to prevent an access port from participating in the spanning tree. If the port still receives a BPDU, it is put in the error-disabled state as a protective measure. This command has three states:

• spanning-tree bpduguard enable —Unconditionally enables BPDU guard on the interface.
• spanning-tree bpduguard disable —Unconditionally disables BPDU guard on the interface.
• no spanning-tree bpduguard —Enables BPDU guard on the interface if the interface is in the operational PortFast state and if you configure the spanning-tree portfast bpduguard default command.

To set the path cost of the interface for Spanning Tree Protocol (STP) calculations, use the spanning-tree cost command in interface configuration mode. To revert to the default value, use the no form of this command.

Syntax Description

Command Default

The default path cost is computed from the bandwidth setting of the interface. Default path costs are:

Command Modes

Interface configuration

Command History

Usage Guidelines

When you specify VLANs associated with a spanning tree instance, you can specify a single VLAN identified by a VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLAN IDs separated by a comma.

When you specify a value for the cost argument, higher values indicate higher costs. This range applies regardless of the protocol type specified.

To display an error message when the switch detects an EtherChannel misconfiguration, use the spanning-tree etherchannel guard misconfig command in global configuration mode. To disable the error message, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

Error messages are displayed.

Command Modes

Global configuration

Command History

Usage Guidelines

When the switch detects an EtherChannel misconfiguration, this error message is displayed:

PM-4-ERR_DISABLE: Channel-misconfig error detected on [chars], putting [chars] in err-disable state.

To determine which local ports are involved in the misconfiguration, enter the show interfaces status err-disabled command. To check the EtherChannel configuration on the remote device, enter the show etherchannel summary command on the remote device.

After you correct the configuration, enter the shutdown and the no shutdown commands on the associated port-channel interface.

To enable extended system identification, use the spanning-tree extend system-id command in global configuration mode. To disable extended system identification, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

The extended system ID is enabled.

Command Modes

Global configuration

Command History

Usage Guidelines

The 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 or multiple spanning-tree instance. Because a switch stack appears as a single switch to the rest of the network, all switches in the stack use the same bridge ID for a given spanning tree. If the stack master fails, the stack members recalculate their bridge IDs of all running spanning trees based on the new MAC address of the stack master.

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.

If your network consists of switches that do not support the extended system ID and switches that do support it, 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.

To enable or disable root-guard mode or loop-guard mode on the VLANs associated with an interface, use the spanning-tree guard command in interface configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

Root-guard mode is disabled.

Loop-guard mode is configured according to the spanning-tree loopguard default command in global configuration mode.

Command Modes

Interface configuration

Usage Guidelines

You can enable root guard or loop guard when the switch is operating in the per-VLAN spanning-tree plus (PVST+), rapid-PVST+, or the multiple spanning-tree (MST) mode.

You cannot enable both root guard and loop guard at the same time.

Use the spanning-tree guard loop command to override the setting of the spanning-tree loop guard default setting.

When root guard is enabled, if spanning-tree calculations cause an interface to be selected as the root port, the interface transitions to the root-inconsistent (blocked) state to prevent the switch from becoming the root switch or from being in the path to the root. The root port provides the best path from the switch to the root switch.

When the no spanning-tree guard or the no spanning-tree guard none command is entered, root guard is disabled for all VLANs on the selected interface. If this interface is in the root-inconsistent (blocked) state, it automatically transitions to the listening state.

Do not enable root guard on interfaces that will be used by the UplinkFast feature. With UplinkFast, the backup interfaces (in the blocked state) replace the root port in the case of a failure. However, if root guard is also enabled, all the backup interfaces used by the UplinkFast feature are placed in the root-inconsistent state (blocked) and are prevented from reaching the forwarding state. The UplinkFast feature is not available when the switch is operating in the rapid-PVST+ or MST mode.

To configure a link type for a port, use the spanning-tree link-type command in the interface configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

Link type is automatically derived from the duplex setting unless you explicitly configure the link type.

Command Modes

Interface configuration

Command History

Usage Guidelines

Rapid Spanning Tree Protocol Plus (RSTP+) fast transition works only on point-to-point links between two bridges.

By default, the switch derives the link type of a port from the duplex mode. A full-duplex port is considered as a point-to-point link while a half-duplex configuration is assumed to be on a shared link.

If you designate a port as a shared link, RSTP+ fast transition is forbidden, regardless of the duplex setting.

To enable loop guard as a default on all ports of a given bridge, use the spanning-tree loopguard default command in global configuration mode. To disable loop guard, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

Loop guard is disabled.

Command Modes

Global configuration

Command History

Usage Guidelines

Loop guard provides additional security in the bridge network. Loop guard prevents alternate or root ports from becoming the designated port due to a failure that could lead to a unidirectional link.

Loop guard operates only on ports that are considered point-to-point by the spanning tree.

The individual loop-guard port configuration overrides this command.

To switch between per-VLAN Spanning Tree+ (PVST+), Rapid-PVST+, and Multiple Spanning Tree (MST) modes, use the spanning-tree mode command in global configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

The default mode is PVST+.

Command Modes

Global configuration

Command History

Usage Guidelines

Only one mode can be active at a time.

All stack members run the same spanning-tree mode.

Caution

Be careful when using the spanning-tree mode command to switch between PVST+, Rapid-PVST+, and MST modes. When you enter the command, all spanning-tree instances are stopped for the previous mode and are restarted in the new mode. Using this command may cause disruption of user traffic.

To enter MST-configuration mode, use the spanning-tree mst configuration command in global configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

The default value for the Multiple Spanning Tree (MST) configuration is the default value for all its parameters:

• No VLANs are mapped to any MST instance (all VLANs are mapped to the Common and Internal Spanning Tree [CIST] instance).
• The region name is an empty string.
• The revision number is 0.

Command Modes

Global configuration

Command History

Usage Guidelines

You can use these commands for MST configuration:

• abort Exits the MST region configuration mode without applying configuration changes.
• exit Exits the MST region configuration mode and applies all configuration changes.
• instance instance_id vlan vlan_id Maps VLANs to an MST instance. The range for instance IDs is 1 to 4094. The range for VLANs is 1 to 4094. You can specify a single VLAN identified by a VLAN ID number, a range of VLANs separated by a hyphen, or a series of VLANs separated by a comma.
• name name Sets the configuration name. The name string is case sensitive and can be up to 32 characters long.
• no Negates the instance, name and revision commands or sets them to their defaults.
• revision version Sets the configuration revision number. The range is 0 to 65535.
• show [ current | pending Displays the current or pending MST region configuration.

In MST mode, a switch stack supports up to 65 MST instances. The number of VLANs that can be mapped to a particular MST instance is unlimited.

For two or more switches to be in the same MST region, they must have the same VLAN mapping, the same configuration name, and the same configuration revision number.

When you map VLANs to an MST instance, the mapping is incremental, and VLANs specified in the command are added to or removed from the VLANs that were previously mapped. To specify a range, use a hyphen; for example, instance 1 vlan 1-63 maps VLANs 1 to 63 to MST instance 1. To specify a series, use a comma; for example, instance 1 vlan 10, 20, 30 maps VLANs 10, 20, and 30 to MST instance 1.

All VLANs that are not explicitly mapped to an MST instance are mapped to the common and internal spanning tree (CIST) instance (instance 0) and cannot be unmapped from the CIST by using the no form of this command.

Changing an MST-configuration mode parameter can cause connectivity loss. To reduce service disruptions, when you enter MST-configuration mode, make changes to a copy of the current MST configuration. When you have finished editing the configuration, you can apply all the changes at once by using the exit keyword, or you can exit the mode without committing any change to the configuration by using the abort keyword.

To set the path cost of the interface for multiple spanning tree (MST) calculations, use the spanning-tree mst cost command in interface configuration mode. To revert to the default value, use the no form of this command.

Syntax Description

Command Default

The default path cost is computed from the bandwidth setting of the interface. Default path costs are:

Command Modes

Interface configuration

Command History

Usage Guidelines

When you specify a value for the cost argument, higher values indicate higher costs.

To set the forward-delay timer for MST instances, use the spanning-tree mst forward-time command in global configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

The default is 15 seconds.

Command Modes

Global configuration

Command History

To set the hello-time delay timer, use the spanning-tree mst hello-time command in global configuration mode. To return to the default settings, use the no form of this command.

Command Default

The default is 2.

Command Modes

Global configuration

Command History

Usage Guidelines

If you do not specify the hello-time value, the value is calculated from the network diameter.

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.

To set the interval between messages that the spanning tree receives from the root switch, use the spanning-tree mst max-age command in global configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

The default is 20.

Command Modes

Global configuration

Command History

To specify the number of possible hops in the region before a bridge protocol data unit (BPDU) is discarded, use the spanning-tree mst max-hops command in global configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

The default is 20.

Command Modes

Global configuration

Command History

To set the priority for an interface, use the spanning-tree mst port-priority command in interface configuration mode. To revert to the default value, use the no form of this command.

Syntax Description

Command Default

The default is 128.

Command Modes

Interface configuration

Command History

Usage Guidelines

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, the multiple spanning tree (MST) puts the interface with the lowest interface number in the forwarding state and blocks other interfaces.

If the switch is a member of a switch stack, you must use the spanning-tree mst instance_id cost cost command to select an interface to put in the forwarding state.

To configure a port to transmit only prestandard bridge protocol data units (BPDUs), use the spanning-tree mst pre-standard command in interface configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

This command has no arguments or keywords.

Command Default

The default is to automatically detect prestandard neighbors.

Command Modes

Interface configuration

Command History

Usage Guidelines

The port can accept both prestandard and standard BPDUs. If the neighbor types are mismatched, only the common and internal spanning tree (CIST) runs on this interface.

Note	If a switch port is connected to a switch running prestandard Cisco IOS software, you must use the spanning-tree mst pre-standard interface configuration command on the port. If you do not configure the port to send only prestandard BPDUs, the Multiple STP (MSTP) performance might diminish.

When the port is configured to automatically detect prestandard neighbors, the prestandard flag always appears in the show spanning-tree mst commands.

To set the bridge priority for an instance, use the spanning-tree mst priority command in global configuration mode. To return to the default setting, use the no form of this command.

Syntax Description

Command Default

The default is 32768.

Command Modes

Global configuration

Command History

Usage Guidelines

You can set the bridge priority in increments of 4096 only. Valid values are 0, 4096, 8192, 12288, 16384, 20480. 24576, 28672, 32768, 40960, 45056, 49152, 53248, 57344 and 61440.

You can enter instance as a single instance or a range of instances, for example, 0-3,5,7-9.

To designate the primary and secondary root switch and set the timer value for an instance, use the spanning-tree mst root command in global configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

None

Command Modes

Global configuration (config)

Command History

Usage Guidelines

Use this command only on backbone switches. You can enter instance-id as a single instance or a range of instances, for example, 0-3,5,7-9.

When you enter the spanning-tree mst instance-id root command, the software tries to set a high enough priority to make this switch the root of the spanning-tree instance. Because of the extended system ID support, the switch sets the switch priority for the instance to 24576 if this value will cause this switch to become the root for the specified instance. If any root switch for the specified instance has a switch priority lower than 24576, the switch sets its own priority to 4096 less than the lowest switch priority. (4096 is the value of the least-significant bit of a 4-bit switch priority value.)

When you enter the spanning-tree mstinstance-id root secondary command, because of support for the extended system ID, the software changes the switch priority from the default value (32768) to 28672. If the root switch fails, this switch becomes the next root switch (if the other switches in the network use the default switch priority of 32768 and are therefore unlikely to become the root switch).

To set the default path-cost calculation method, use the spanning-tree pathcost method command in global configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

short

Command Modes

Global configuration

Command History

Usage Guidelines

The long path-cost calculation method utilizes all 32 bits for path-cost calculation and yields values in the range of 1 through 200,000,000.

The short path-cost calculation method (16 bits) yields values in the range of 1 through 65535.

To configure an interface priority when two bridges tie for position as the root bridge, use the spanning-treeport-priority command in interface configuration mode. To return to the default value, use the no form of this command.

Syntax Description

Command Default

The port priority is 128.

Command Modes

Interface configuration

Command History

Usage Guidelines

The priority you set breaks the tie.

To enable bridge protocol data unit (BPDU) filtering on PortFast-enabled interfaces, the BDPU guard feature on PortFast-enabled interfaces, or the PortFast feature on all nontrunking interfaces, use the spanning-treeportfast command in global configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

Disabled

Command Modes

Global configuration

Command History

Usage Guidelines

You can enable these features when the switch is operating in the per-VLAN spanning-tree plus (PVST+) rapid-PVST+, or the multiple spanning-tree (MST) mode.

Use the spanning-tree portfast bpdufilter default global configuration command to globally enable BPDU filtering on interfaces that are PortFast-enabled (the interfaces are in a PortFast-operational state). The interfaces still send a few BPDUs at link-up before the switch begins to filter outbound BPDUs. You should globally enable BPDU filtering on a switch so that hosts connected to switch interfaces do not receive BPDUs. If a BPDU is received on a PortFast-enabled interface, the interface loses its PortFast-operational status and BPDU filtering is disabled.

You can override the spanning-tree portfast bpdufilter default command by using the spanning-tree portfast bpdufilter interface command.

Caution

Be careful when using this command. Enabling BPDU filtering on an interface is the same as disabling spanning tree on it and can result in spanning-tree loops.

Use the spanning-tree portfast bpduguard default global configuration command to globally enable BPDU guard on interfaces that are in a PortFast-operational state. In a valid configuration, PortFast-enabled interfaces do not receive BPDUs. Receiving a BPDU on a PortFast-enabled interface signals an invalid configuration, such as the connection of an unauthorized device, and the BPDU guard feature puts the interface in the error-disabled state. The BPDU guard feature provides a secure response to invalid configurations because you must manually put the interface back in service. Use the BPDU guard feature in a service-provider network to prevent an access port from participating in the spanning tree.

You can override the spanning-tree portfast bpduguard default command by using the spanning-tree portfast bpduguard interface command.

Use the spanning-tree portfast default command to globally enable the PortFast feature on all nontrunking interfaces. Configure PortFast only on interfaces that connect to end stations; otherwise, an accidental topology loop could cause a data packet loop and disrupt switch and network operation. A PortFast-enabled interface moves directly to the spanning-tree forwarding state when linkup occurs; it does not wait for the standard forward-delay time.

You can override the spanning-tree portfast default global configuration command by using the spanning-tree portfast interface configuration command. You can use the no spanning-tree portfast default global configuration command to disable PortFast on all interfaces unless they are individually configured with the spanning-tree portfast interface configuration command.

To enable PortFast mode where the interface is immediately put into the forwarding state upon linkup without waiting for the timer to expire, use the spanning-tree portfast command in interface configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

The settings that are configured by the spanning-treeportfastdefault command.

Command Modes

Interface configuration

Command History

Usage Guidelines

You can enable this feature when the switch is operating in the per-VLAN spanning-tree plus (PVST+), Rapid PVST+, or the multiple spanning-tree (MST) mode.

This feature affects all VLANs on the interface.

Use this command only on interfaces that connect to end stations; otherwise, an accidental topology loop could cause a data-packet loop and disrupt the switch and network operation.

To enable PortFast on trunk ports, you must use the spanning-tree portfast trunk interface configuration command. The spanning-tree portfast command is not supported on trunk ports.

An interface with the PortFast feature enabled is moved directly to the spanning-tree forwarding state without the standard forward-time delay.

You can use the spanning-tree portfast default global configuration command to globally enable the PortFast feature on all nontrunking interfaces. Use the spanning-tree portfast interface configuration command to override the global setting.

If you configure the spanning-tree portfast default global configuration command, you can disable PortFast on an interface that is not a trunk interface by using the spanning-tree portfast disable interface configuration command.

To specify the transmit hold count, use the spanning-tree transmit hold-count command in global configuration mode. To return to the default settings, use the no form of this command.

Syntax Description

Command Default

The default is 6.

Command Modes

Global configuration

Command History

Usage Guidelines

This command is supported on all spanning-tree modes.

The transmit hold count determines the number of BPDUs that can be sent before pausing for 1 second.

Note	Increasing the transmit-hold count value can have a significant impact on CPU utilization, especially in Rapid Per-VLAN Spanning Tree (PVST+) mode. Decreasing this value might result in slow convergence. We recommend that you used the default setting.