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Difficulty: Very hardSTP Port States and PortFast Benefits

A network operations center (NOC) engineer observes that workstations attached to switch SW-CORE-01 experience a 30-second initial DHCP request timeout upon link activation before successfully receiving an IP address. Interface GigabitEthernet1/0/5 is currently configured as a standard 802.1D access port. Which operational mechanism explains why enabling Cisco STP PortFast resolves this DHCP timeout issue, and what is its specific effect on STP Topology Change Notifications (TCNs)?

  1. A
    PortFast completely disables BPDU transmission and reception on the interface, eliminating STP convergence overhead and preventing TCNs from being forwarded to the root bridge.
  2. PortFast allows the interface to bypass the Listening and Learning states to enter Forwarding immediately upon link up, while suppressing TCN generation when the edge port link status changes.Answer
  3. C
    PortFast modifies the local STP bridge priority increment to force immediate convergence, triggering a direct TCN flood to clear downstream MAC address tables.
  4. D
    PortFast transitions the interface directly from Blocking to Learning state, skipping only the Listening timer while continuing to transmit standard TCN BPDUs.

Answer

PortFast allows the interface to bypass the Listening and Learning states to enter Forwarding immediately upon link up, while suppressing TCN generation when the edge port link status changes.
The correct answer identifies that PortFast allows access ports connected to end hosts to transition directly to the Forwarding state without waiting through the 15-second Listening and 15-second Learning states (a total 30-second convergence delay). This eliminates initialization delays for client services such as DHCP. In addition, link state fluctuations on a PortFast-enabled port do not trigger Topology Change Notifications (TCNs), avoiding network-wide MAC address table flushing.

Step-by-Step Solution

1
Analyze standard 802.1D STP convergence behavior on access ports.
Standard 802.1D transitions through Blocking -> Listening (15s) -> Learning (15s) -> Forwarding (total 30s delay).
During the 30-second delay, frames are dropped, causing client protocols like DHCP to time out while waiting for link establishment.
2
Determine the primary operational transition bypass provided by PortFast.
PortFast moves an edge interface immediately from Blocking to Forwarding upon link bring-up.
By bypassing Listening and Learning states, connectivity is instantaneous, eliminating host initialization timeouts.
3
Analyze the secondary benefit regarding Spanning Tree Topology Change Notifications (TCNs).
PortFast suppresses TCN generation on edge link state changes.
End-user PC power cycles or disconnects should not force switches across the network to shorten their MAC table aging timers.

Key Concept

STP PortFast immediate forwarding transition and TCN suppression on access ports
Estimated Time:2m 0s
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Topics

STP Port States and PortFast Benefits

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