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Cisco Catalyst Blade Switch 3120 for HP Command Reference
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Chapter 2 Cisco Catalyst Blade Switch 3120 for HP Cisco IOS Commands
spanning-tree guard
spanning-tree guard
Use the
spanning-tree guard
interface configuration command on the switch stack or on a standalone
switch to enable root guard or loop guard on all the VLANs associated with the selected interface. Root
guard restricts which interface is allowed to be the spanning-tree root port or the path-to-the root for the
switch. Loop guard prevents alternate or root ports from becoming designated ports when a failure
creates a unidirectional link. Use the
no
form of this command to return to the default setting.
spanning-tree guard
{
loop
|
none | root
}
no spanning-tree guard
Syntax Description
Defaults
Root guard is disabled.
Loop guard is configured according to the
spanning-tree loopguard default
global configuration
command (globally disabled).
Command Modes
Interface configuration
Command History
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.
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 customer’s switch
from becoming the root switch or 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 prevented from reaching the forwarding state. The UplinkFast
feature is not available when the switch is operating in the rapid-PVST+ or MST mode.
Loop guard is most effective when it is configured on the entire switched network. When the switch is
operating in PVST+ or rapid-PVST+ mode, loop guard prevents alternate and root ports from becoming
designated ports, and spanning tree does not send bridge protocol data units (BPDUs) on root or alternate
loop
Enable loop guard.
none
Disable root guard or loop guard.
root
Enable root guard.
Release
Modification
12.2(40)EX1
This command was introduced.
