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Difficulty: HardSTP Port States and PortFast Benefits

An administrator executes the following command on a Cisco Catalyst switch port connected to an unmanaged access switch:

`Switch(config-if)# spanning-tree portfast`

Shortly after, a transient Layer 2 loop brings down the network segment before STP can converge. Which operational characteristic of Cisco PortFast directly allowed this loop to form?

  1. PortFast transitions the interface immediately from Blocking to Forwarding, bypassing the Listening and Learning states that normally detect loops.Answer
  2. B
    PortFast suppresses the transmission and reception of Bridge Protocol Data Units (BPDUs) across the configured interface.
  3. C
    PortFast forces the local switch interface to alter its bridge priority calculation to 0, ensuring it becomes the Root Bridge for that segment.
  4. D
    PortFast automatically disables 802.1Q encapsulation on the port, causing frames from different VLANs to merge into a single broadcast domain.

Answer

PortFast transitions the interface immediately from Blocking to Forwarding, bypassing the Listening and Learning states that normally detect loops.
Enabling PortFast on an access port allows it to immediately transition from Blocking directly into the Forwarding state, bypassing the 15-second Listening state and 15-second Learning state. When connected to another switch (even an unmanaged one), this immediate transition to Forwarding creates a transient or permanent Layer 2 loop before STP has the chance to listen for BPDUs and move redundant ports into the Blocking state.

Step-by-Step Solution

1
Analyze standard 802.1D STP state transitions for a newly connected link.
Standard ports move through Blocking → Listening (15 seconds) → Learning (15 seconds) → Forwarding (total delay of 30 seconds).
The Listening and Learning states allow the switch to receive BPDUs and build MAC address tables to detect potential loops before user traffic is forwarded.
2
Evaluate the behavior of enabling PortFast on an interface.
PortFast skips Listening and Learning, bringing the interface directly into the Forwarding state upon link-up.
This is intended for edge end-user devices (workstations, printers) that cannot cause Layer 2 loops.
3
Determine the impact of enabling PortFast on a switch-to-switch link (or link connected to an unmanaged switch).
Because the port immediately enters Forwarding state, broadcast traffic and loops occur instantly before BPDUs can be evaluated to block the redundant path.
Misapplying PortFast on non-edge ports bypasses loop-prevention safeguards.

Key Concept

Cisco PortFast operational mechanism and misconfiguration risks on switch interconnects.
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Topics

STP Port States and PortFast Benefits

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