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Difficulty: MediumSTP Port States and PortFast Benefits

An engineer is diagnosing network initialization delays for point-of-sale terminals connected to a switch. Output from the switch CLI reveals the following interface status:

Switch# show spanning-tree interface gigabitEthernet 1/0/5

Vlan Role Sts Cost Prio.Nbr Type
------------------- ---- --- --------- -------- --------------------------------
VLAN0020 Desg LRN 4 128.5 P2p

Which primary operational benefit is provided by enabling Cisco PortFast on this access port?

  1. A
    It lowers the STP bridge priority in increments of 4096 to ensure the switch becomes the Root Bridge for VLAN 20.
  2. It allows the port to immediately transition from Blocking to Forwarding, avoiding the standard delays associated with Listening and Learning states.Answer
  3. C
    It enables rapid Spanning Tree convergence across switch-to-switch trunk links by suppressing BPDU transmissions.
  4. D
    It isolates broadcast domains at Layer 2 by automatically converting the switchport into an IP routed interface.

Answer

Enabling PortFast allows an access port connected to an end device to bypass the 802.1D Listening (15s) and Learning (15s) states, transitioning directly from Blocking to Forwarding upon link activation.
In standard 802.1D Spanning Tree Protocol, an interface transitions through Listening (15 seconds) and Learning (15 seconds) states before reaching Forwarding. The CLI output shows the port in the Learning ('LRN') state. Enabling Cisco PortFast on edge ports connected to single end-user devices allows the port to immediately enter the Forwarding state upon linkup, preventing boot and network assignment timeouts.

Step-by-Step Solution

1
Analyze the CLI output for interface status
The interface status code 'LRN' indicates the port is currently in the 802.1D STP Learning state.
Standard 802.1D STP transitions ports through Blocking (20s max age), Listening (15s forward delay), and Learning (15s forward delay) before reaching Forwarding.
2
Determine the impact of PortFast on port state transitions
PortFast immediately places access ports into the Forwarding state upon link up, bypassing Listening and Learning.
End-user devices do not create Layer 2 loops, so waiting 30 seconds for Listening and Learning causes application timeouts (such as DHCP or POS terminal boot routines).

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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