Examkin

Question

Difficulty: HardSTP Port States and PortFast Benefits

During a network audit, a network engineer discovers that `spanning-tree portfast default` is enabled globally on an access switch. A junior technician subsequently connects an unmanaged switch to one of the access ports. Assuming BPDU Guard is NOT configured on the port, what occurs when the Cisco switch interface receives a Spanning Tree Protocol (STP) BPDU from the unmanaged switch?

  1. The interface immediately loses its PortFast operational status, transitions to the Blocking state, and moves through standard IEEE 802.1D port states.Answer
  2. B
    The interface permanently suppresses the incoming BPDUs and remains in the Forwarding state because PortFast disables BPDU processing.
  3. C
    The interface transitions into an err-disabled state and automatically modifies the switch bridge priority by an increment of 4096.
  4. D
    The interface bypasses the Blocking state and immediately enters the Learning state to accelerate convergence while processing the BPDU.

Answer

The interface immediately loses its PortFast operational status, transitions to the Blocking state, and moves through standard IEEE 802.1D port states.
Cisco PortFast allows end-user host ports to bypass the 30-second STP listening and learning delays and enter the forwarding state immediately. However, PortFast does not disable BPDU processing. If a PortFast-enabled interface receives an STP BPDU (indicating a switch or hub is attached), the interface loses its PortFast status and reverts to standard 802.1D STP operation, transitioning to the Blocking state to safely evaluate network topology and prevent bridging loops.

Step-by-Step Solution

1
Analyze PortFast behavior upon receiving BPDUs without BPDU Guard
PortFast allows an access port to immediately move from Blocking to Forwarding, skipping Listening and Learning states. However, it does not stop the port from sending or receiving BPDUs.
PortFast is designed strictly for edge ports connected to single end-user devices.
2
Evaluate the interface state transition when a BPDU arrives
When a BPDU is received on a PortFast-enabled interface without BPDU Guard active, the switch detects another bridge device. PortFast operational status is immediately revoked, and the port falls back to standard IEEE 802.1D state transitions.
Falling back to normal STP port states (Blocking -> Listening -> Learning -> Forwarding) protects the network from potential Layer 2 loops.

Key Concept

PortFast BPDU Handling and Operational State Reversion
Rate this question

Topics

STP Port States and PortFast Benefits

More questions from this topic