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Difficulty: HardSTP Port States and PortFast Benefits

An engineer executes a verification command on a Cisco Catalyst switch interface connected to a desktop computer:

text
Switch# show spanning-tree interface GigabitEthernet0/1 detail
Port 1 (GigabitEthernet0/1) of VLAN0010 is forwarding
Port path cost 4, Port priority 128.1, Port Identifier 128.1.
Designated root has priority 32778, address 0050.56be.1100
Designated bridge has priority 32778, address 0050.56be.1100
Designated port id is 128.1, designated path cost 0
Timers: message age 0, forward delay 0, hold 0
The port is in the portfast mode
Link type is point-to-point by default
BPDU: sent 1420, received 0

Based on the command output, what will occur in the Spanning Tree Protocol domain when the desktop connected to GigabitEthernet0/1 undergoes a link state flap (disconnects and reconnects)?

  1. The interface immediately transitions to the Forwarding state upon link-up without generating Topology Change Notifications (TCNs) to upstream switches.Answer
  2. B
    The interface enters the Listening state for 15 seconds while transmitting TCN BPDUs to force all switches in the VLAN to age out MAC address table entries.
  3. C
    The interface transitions through the Learning state to rebuild its local MAC address table before allowing unicast frame forwarding.
  4. D
    The interface stops sending BPDUs and transitions to the Disabled state because BPDU sent count exceeds the received count.

Answer

The interface immediately transitions to the Forwarding state upon link-up without generating Topology Change Notifications (TCNs) to upstream switches.
When Cisco STP PortFast is enabled on an access interface, it provides two main benefits: immediate transition to the Forwarding state (bypassing the 15-second Listening and 15-second Learning states) and suppression of Topology Change Notification (TCN) messages. Because host connections frequently open and close applications or restart, suppressing TCNs prevents the switch from notifying the Root Bridge and forcing all domain switches to temporarily shorten their MAC address table aging timers to Forward Delay (15 seconds).

Step-by-Step Solution

1
Analyze the CLI command output for operational features.
The output explicitly confirms 'The port is in the portfast mode'.
PortFast is enabled on GigabitEthernet0/1, which modifies default 802.1D STP state transition and notification logic.
2
Determine the impact of PortFast on port state transitions during link initiation.
The interface bypasses the 15-second Listening and 15-second Learning timer delays.
PortFast allows an access port connected to a host to move immediately from Blocking to Forwarding.
3
Determine the impact of PortFast on STP domain notifications during link flaps.
No Topology Change Notification (TCN) BPDUs are generated by the switch for this interface event.
Edge port state changes do not represent network topology alterations, so suppressing TCNs prevents unnecessary MAC table timer reductions across the entire switching domain.

Key Concept

Cisco STP PortFast Operational Mechanics and TCN Suppression
Estimated Time:1m 30s
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Topics

STP Port States and PortFast Benefits

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