Cisco ME 3600X, Command Reference Manual

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Cisco ME 3800X and ME 3600X Switch Command Reference
OL-28238-01
Chapter
spanning-tree guard
spanning-tree guard
To enable root guard or loop guard on all the VLANs associated with the selected port, use the
spanning-tree guard command in interface configuration mode. 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. To
return to the default setting, use the
no form of this command.
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+), the 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 NNI. If this interface is in the root-inconsistent (blocked) state,
it automatically transitions to the listening state.
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
ports. When the switch is operating in MST mode, BPDUs are not sent on nonboundary interfaces if the
interface is blocked by loop guard in all MST instances. On a boundary interface, loop guard blocks the
interface in all MST instances.
To disable root guard or loop guard, use the spanning-tree guard none interface configuration
command on an STP interface. You cannot enable both root guard and loop guard at the same time.
loop Enabled loop guard.
none Disabled root guard or loop guard.
root Enabled root guard.
Release Modification
12.2(52)EY This command was introduced.