400-007 Question #66: Real Exam Question with Answer & Explanation

Question

Options

• APortFast does not generate a spanning tree topology change hen a station on a port is connected
• BPortFast disables spanning tree on the port, which puts the port into the forwarding state
• CPortFast allows small, unmanaged switches to be plugged into ports of access switches without
• DPortFast detects one-way communications on the physical port, which prevents switch loops
• EPortFast prevents switch loops that are caused by a unidirectional point to point link condition on
• FPortFast prevents switched traffic from traversing suboptimal paths on the network

Explanation

PortFast suppresses Topology Change Notifications (TCNs) when end stations connect or disconnect, preventing unnecessary MAC address table flushes across the network.

Common mistakes.

• B. PortFast does not disable STP on the port - STP BPDUs are still processed, and receiving a BPDU on a PortFast port causes it to lose PortFast behavior; the port simply skips the listening and learning states.
• C. Allowing unmanaged switches into PortFast-enabled ports is actually dangerous, not a benefit, because a switch can introduce a loop; BPDU Guard is the companion feature used to protect against this.
• D. Detecting one-way communications on a physical link is the function of UDLD (UniDirectional Link Detection), not PortFast.
• E. Preventing loops caused by unidirectional point-to-point link conditions is the purpose of UDLD, not PortFast.
• F. Preventing suboptimal path traversal is a function of STP root bridge placement or loop guard, not PortFast.

Concept tested. PortFast TCN suppression behavior in STP

Reference. https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html

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