To enable or disable UDLD and loop guard, use these commands:
– 
Configuring UDLD
UDLD is used when a link should be shut down
because of a hardware failure that is causing unidirectional communication. In
an EtherChannel bundle, UDLD will shut down only the physical link that has
failed.

UDLD can
be enabled globally for all fiber interfaces or on a per interface basis.
Enable UDLD on an Interface
To enable UDLD on an interface use
the following command:
Switch(config-if)#udld
enable
Enable UDLD Globally
To enable UDLD
globally on all fiber-optic interfaces, use the following command:
Switch(config)#udld enable
Verify and Resetting UDLD
Interfaces will be shut down by UDLD.
To reset all interfaces that have been shut down by UDLD, enter this command:

Switch#udld reset
To verify the
UDLD configuration for an interface, enter this command:
Switch#show udld interface
Example: Displaying the UDLD State
This example shows how to
display the UDLD state for a single interface.

Switch#show udld
GigabitEthernet2/2
Configuring Loop Guard
Loop guard is enabled on a
per-port basis. When loop guard is enabled, it is automatically applied to all
of the active VLAN instances to which that port belongs. When you disable loop
guard, it is disabled for the specified ports. Disabling loop guard moves all
loop-inconsistent ports to the listening state. If loop guard is enabled on an
EtherChannel interface, the entire channel will be blocked for a particular
VLAN. This is because EtherChannel is regarded as one logical port from an STP
point of view.
Loop guard should be enabled on the root port and the alternate ports on
access switches.
Enable Loop Guard on an Interface 
To enable
loop guard on a specific interface, issue this command:
Switch(config)#spantree guard loop
mod/port
To disable loop guard, issue this
command:
Switch(config)#spantree guard none
mod/port
Enabling loop guard will disable root guard,
if root guard is currently enabled on the ports.
Enable Loop Guard Globally
Loop guard can be enabled globally on
a switch for all point-to-point links. A full-duplex link is considered to be a
point-to-point link. The status of loop guard can be changed on an interface
even if the feature has been enabled globally.
To enable loop guard
globally, issue this command:
Switch(config)#spantree global-default loopguard
enable
To globally disable loop guard, issue this
command:
Switch(config)#spantree global-default loopguard
disable
Verifying the Loop Guard Status
To verify the loop guard status,
issue this command:
Switch#show spantree guard
mod/port | vlan
For example:
Switch#show spantree guard
3/13
| Port | VLAN | Port-State | Guard Type |
| ------- | ------- | ------------------- | ---------------- |
| 3/13 | 2 | forwarding | loop |