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Difficulty: Very hardSTP Port States and PortFast Benefits

An engineer is troubleshooting a switch port running classic 802.1D Spanning Tree Protocol (STP) that connects to an access host. The current interface state is shown below:

text
Switch# show spanning-tree interface GigabitEthernet 0/1
Vlan Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0010 Desg LRN 4 128.1 P2p

If the network administrator configures `spanning-tree portfast` on interface GigabitEthernet 0/1 and the link subsequently flaps, which statement accurately describes the operational state transitions and spanning-tree behaviors of this interface?

  1. The interface immediately transitions upon link-up from Blocking directly to Forwarding, skipping Listening and Learning, and it does not trigger a Topology Change Notification (TCN) when the link flaps.Answer
  2. B
    The interface transitions immediately to Forwarding but suppresses all outbound BPDUs, making PortFast safe to deploy on switch-to-switch trunk interconnections.
  3. C
    The interface bypasses the Listening state but remains in the Learning state for 15 seconds to populate the MAC address table while calculating the system ID extension increment of 4096.
  4. D
    The interface transitions immediately to Forwarding, but every link flap forces the switch to transmit a TCN BPDU out its root port to clear MAC address tables across the broadcast domain.

Answer

The interface immediately transitions upon link-up from Blocking directly to Forwarding, skipping Listening and Learning, and it does not trigger a Topology Change Notification (TCN) when the link flaps.
Enabling Cisco PortFast on an access interface causes it to transition directly from Blocking to Forwarding immediately upon link up, completely bypassing the 15-second Listening and 15-second Learning states. In addition, link flaps on PortFast-enabled ports do not send Topology Change Notifications (TCNs) to the Root Bridge, avoiding global MAC address table aging flushes.

Step-by-Step Solution

1
Analyze the initial interface output status.
The command output shows interface GigabitEthernet 0/1 in the Designated role and LRN (Learning) state.
Standard 802.1D STP transitions through Blocking (20s max age) -> Listening (15s forward delay) -> Learning (15s forward delay) -> Forwarding (total convergence delay of 30-50 seconds).
2
Evaluate the impact of enabling Cisco PortFast on an access interface.
PortFast bypasses the Listening and Learning states, placing the port into Forwarding immediately when link-up occurs.
End-user hosts (workstations, servers, printers) do not create Layer 2 loops and require rapid link activation to prevent DHCP lease timeouts or PXE boot failures.
3
Determine the Topology Change Notification (TCN) behavior associated with PortFast.
Link up/down events on PortFast-enabled ports do not generate TCNs.
Preventing TCN generation eliminates unnecessary MAC address table timer reductions (aging down to forward delay) across the switches in the STP domain whenever end-user PCs reboot or disconnect.

Key Concept

STP Port States and PortFast Benefits
Estimated Time:1m 30s
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Topics

STP Port States and PortFast Benefits

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