Which of the following is true regarding loop guard? (Select the best answer.)
- Loop guard should be used in conjunction with root guard.
- Loop guard should be used in conjunction with PortFast.
- Loop guard places inconsistent ports into the blocking state.
- Loop guard is used to disable ports that receive BPDUs.
Loop guard places inconsistent ports into the blocking state. Loop guard prevents a switch port from transitioning to the forwarding state when it stops receiving bridge protocol data units (BPDUs)? this prevents switching loops from occurring. A port configured with loop guard that stops receiving BPDUs will be put into the loopinconsistent state, as shown in the following output:
%SPANTREE4LOOPGUARDBLOCK: No BPDUs were received on port 0/1 in vlan 4. Moved to loop inconsistent state
After the port starts receiving BPDUs again, loop guard enables the port to transition through the normal Spanning Tree Protocol (STP) states.
Loop guard should not be used in conjunction with root guard. Root guard is used to prevent newly introduced switches from being elected the new root. This allows administrators to maintain control over which switch is the root. When STP is used, the device with the lowest bridge priority is elected the root. If an additional device is added to the network with a lower priority than the current root, it will send superior BPDUs and be elected the new root. However, this could cause the network to reconfigure in unintended ways. To prevent this, root guard can be applied. If root guard is enabled on a loop guard-enabled port, loop guard will be automatically disabled.
Loop guard should not be used in conjunction with PortFast. PortFast reduces convergence time by immediately placing edge ports into a forwarding state. PortFast is recommended for ports that connect to enduser devices, such as desktop computers. In addition, PortFast cannot be used with loop guard. If PortFast is enabled on a loop guard-enabled port, loop guard will be automatically disabled.
Loop guard is not used to disable ports that receive BPDUs. Instead, BPDU guard is used to disable ports that erroneously receive BPDUs. BPDU guard is applied to edge ports that have PortFast enabled. Because PortFast automatically places ports into a forwarding state, a switch that has been connected to a PortFastenabled port could cause switching loops. However, when BPDU guard is applied, the receipt of a BPDU on a port with BPDU guard enabled will result in the port being placed into an errdisable state, which prevents loops from occurring.