352-001 · Question #602
Which effect of designing a Layer 2 network using the PortFast fast feature with PVST+ is true?
The correct answer is D. It moves the switch port directly to the forwarding state. PortFast causes a switch access port to skip the STP listening and learning states and transition immediately to the forwarding state upon link-up.
Question
Which effect of designing a Layer 2 network using the PortFast fast feature with PVST+ is true?
Options
- AIt shuts down the port when receiving the superior BPDU
- BIt accelerates the network convergence on the trunk uplinks
- CIn combination with BPDU filtering, it causes the switch port to stay in the forwarding state
- DIt moves the switch port directly to the forwarding state
How the community answered
(14 responses)- B7% (1)
- C7% (1)
- D86% (12)
Why each option
PortFast causes a switch access port to skip the STP listening and learning states and transition immediately to the forwarding state upon link-up.
Shutting down a port upon receiving a superior BPDU is the behavior of BPDU Guard, a separate feature commonly deployed alongside PortFast to protect against accidental switch connections but not part of PortFast itself.
PortFast is designed exclusively for access ports connected to end hosts - applying it to trunk uplinks is a misconfiguration that can cause bridging loops, and it does not accelerate convergence on uplinks.
A port remaining in forwarding state despite receiving BPDUs is the combined effect of BPDU filtering suppressing STP processing, not the standalone behavior of the PortFast feature alone.
PortFast bypasses the standard STP state machine transitions (blocking, listening, learning) and places the port directly into forwarding state as soon as the link comes up, eliminating the 30-50 second delay that end hosts would otherwise experience before sending traffic. This is safe only on ports connected to end hosts that will never generate topology change notifications or form STP loops.
Concept tested: STP PortFast direct forwarding state transition on access ports
Source: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/17-3/configuration_guide/lyr2/b_173_lyr2_9300_cg/configuring_optional_spanning_tree_features.html
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