Which of the following features can cause a switch port to enter the errdisable state? (Select the best answer.)
A.
BPDU guard
B.
PortFast
C.
root guard
D.
loop guard
Explanation:
The BPDU guard feature can cause a switch port to enter the errdisable state. The BPDU guard feature should
be enabled on PortFastenabled ports so that BPDU guard can prevent a rogue switch from modifying the
Spanning Tree Protocol (STP) topology and becoming the root bridge. When such a port receives a bridge
protocol data unit (BPDU), BPDU guard immediately puts that port into the errdisable state and shuts down the
port. The port must then be manually reenabled, or it can be recovered automatically through the errdisable
timeout function. BPDU guard should not be enabled on ports that are connected to other switches.
PortFast is a feature that should be used only on switch ports that are connected to end devices, such as user
workstations or print devices. Because PortFast immediately transitions a port to the STP forwarding state,
skipping over the listening and learning states, steps should be taken to ensure that a switch that is
inadvertently or intentionally connected to the port cannot influence the STP topology.
The root guard feature, when enabled on a port, prevents superior BPDUs received on a neighbor switch
connected to that port from becoming the root bridge. If superior BPDUs are received on a port enabled with
root guard, the port enters the rootinconsistent state and no data will flow through that port until the port stops
receiving superior BPDUs.
The loop guard feature prevents nondesignated ports from inadvertently forming bridging loops if the steady
flow of BPDUs is interrupted. When the port stops receiving BPDUs, loop guard puts the port into the
loopinconsistent state, which keeps the port in a blocking state. After the port starts receiving BPDUs again,
loop guard enables the port to transition through the normal STP states.Cisco: Spanning Tree PortFast BPDU Guard Enhancement: Feature Description