Spanning-Tree Guard - Cisco Catalyst 2960 Command Reference Manual

spanning-tree guard

spanning-tree guard
Use the spanning-tree guard interface configuration command 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.
Syntax Description
loop
none
root
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 Release
12.2(25)FX
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
Catalyst 2960 Switch Command Reference
2-652
spanning-tree guard {loop | none | root}
no spanning-tree guard
Enable loop guard.
Disable root guard or loop guard.
Enable root guard.
Modification
This command was introduced.
Chapter 2 Catalyst 3750 Switch Cisco IOS Commands
OL-8604-03