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Difficulty: MediumSTP Port States and PortFast Benefits

A network engineer executes the command `spanning-tree portfast trunk` on interface GigabitEthernet0/1 of a Cisco Catalyst switch. This interface acts as an active 802.1Q trunk link connected to another switch. Which operational risk is introduced by enabling PortFast on this trunk interface?

  1. The interface immediately transitions to the Forwarding state upon link setup, creating a risk of transient Layer 2 loops.Answer
  2. B
    The switch stops transmitting and receiving BPDUs on the interface, permanently suppressing Spanning Tree Protocol operation.
  3. C
    The connected switch automatically adjusts its Bridge Priority by subtracting increments of 4096 to claim Root Bridge status.
  4. D
    The interface terminates all broadcast traffic and converts the single broadcast domain into isolated collision domains per VLAN.

Answer

The interface immediately transitions to the Forwarding state upon link setup, creating a risk of transient Layer 2 loops.
Enabling PortFast forces a switch port to bypass the 802.1D Listening and Learning states (30 seconds total default delay) and immediately enter the Forwarding state when the link becomes active. When applied to switch-to-switch interswitch connections, this behavior exposes the network to temporary bridging loops and broadcast storms during interface startup.

Step-by-Step Solution

1
Analyze the function of Cisco PortFast.
PortFast allows an interface to bypass the 802.1D STP Listening (15s) and Learning (15s) states, transitioning immediately from Blocking to Forwarding.
It is designed for edge ports connected to single end-user devices (like PCs or printers) where topology loops cannot occur.
2
Evaluate the risk of applying PortFast to switch-to-switch trunk links.
When applied to interswitch links using `spanning-tree portfast trunk`, the port skips loop-prevention delay states, creating potential transient broadcast storms/loops before STP converges.
Switch-to-switch links form topology loops that depend on STP Listening and Learning states to determine block/forward roles.

Key Concept

STP PortFast operational behavior and interswitch configuration risk
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Topics

STP Port States and PortFast Benefits

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