Which of the following commands can be issued without interfering with the operation of loop guard?
A.
Switch(config-if)#spanning-tree guard root
B.
Switch(config-if)#spanning-tree portfast
C.
Switch(config-if)#switchport mode trunk
D.
Switch(config-if)#switchport mode access
Explanation:
You can use loop guard to prevent alternate or root ports from becoming designated ports because of a failure that leads to a unidirectional link. This feature is most effective when it is enabled on the entire switched network. Loop guard prevents alternate and root ports from becoming designated ports, and spanning tree does not send BPDUs on root or alternate ports.You can enable this feature by using the spanning-tree loopguard default global configuration command.
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 BPDUs on root or alternate ports.
When the switch is operating in MST mode, BPDUs are not sent on nonboundary ports only if the interface is blocked by loop guard in all MST instances. On a boundary port, loop guard blocks the interface in all MST instances.
True answer is B:
Switch(config-if)#spanning-tree portfast
Switch(config-if)#switchport mode trunk
Switch(config-if)#switchport mode access
These commands change a port status to Down and then to Up or STP Blocked or Inconsistent. They are have effect to existing operation of loop guard.
Switch(config-if)#spanning-tree guard root
You cannot enable loop guard and root guard on a port at the same time.
Then this command has effect to existing operation of loop guard.
Only PortFast command you can enter without any effect to existing operation of loop guard. Because you cannot enable PortFast on loop guard-enabled ports, it is ignored.
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ANSWER: C
UDLD and Loop Guard: Protecting trunks
Both UniDirectional Link Detection (UDLD) and Loop Guard protect a switch trunk port from causing loops. Both features prevent switch ports from errantly moving from a blocking to a forwarding state when a unidirectional link exists in the network.
Unidirectional links are simply links for which one of the two transmission paths on the link has failed, but not both. This can happen as a result of miscabling, cutting one fiber cable, unplugging one fiber, GBIC problems, or other reasons
(Reference: CCIE Routing and Switching Exam Certification Guide)
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