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Difficulty: MediumSTP Port States and PortFast Benefits

A network administrator connects a PC to a Cisco switch access port configured with standard IEEE 802.1D Spanning Tree Protocol (STP) without PortFast enabled. Place the STP port states in the correct chronological order from the moment the physical link comes up until the port actively processes end-user network traffic.

  1. 1Blocking State: Discards incoming user frames and does not learn MAC addresses; processes received BPDUs to prevent initial bridging loops.
  2. 2Listening State: Transmits and receives BPDUs to participate in Root Bridge election and port role determination; discards user frames and does not learn MAC addresses.
  3. 3Learning State: Continues processing BPDUs and begins populating the MAC address table from incoming frame source addresses; does not forward user data frames.
  4. 4Forwarding State: Actively sends and receives user data frames, updates the MAC address table, and processes BPDUs.

Answer

The correct chronological order of 802.1D STP port state transitions from link initiation to active forwarding is: Blocking State, Listening State, Learning State, and Forwarding State.
Under standard IEEE 802.1D Spanning Tree Protocol, an access interface without PortFast undergoes a 30-second convergence delay through specific sequential states: Blocking state (loop prevention), Listening state (15s to exchange BPDUs and determine port roles), Learning state (15s to populate MAC tables), and finally Forwarding state (active user traffic processing).

Step-by-Step Solution

1
Identify the starting state when the physical interface transitions to up.
The port initializes in the Blocking state to prevent layer 2 loops.
Before topology calculations occur, all data frame processing and MAC learning are disabled.
2
Determine the state transition after the initial blocking evaluation.
The port moves to the Listening state for the duration of the Forward Delay timer (15 seconds).
In Listening, the switch participates in BPDUs exchange to determine spanning-tree topology roles without learning MAC addresses or forwarding frames.
3
Determine the state transition after port roles are settled.
The port enters the Learning state for a second Forward Delay interval (15 seconds).
In Learning, the port populates its MAC address table from frame headers to avoid excessive unicast flooding once forwarding begins.
4
Identify the final operational state.
The port transitions to the Forwarding state after completing both Forward Delay periods (30 seconds total convergence delay).
The port is now permitted to actively forward user data plane frames.

Key Concept

IEEE 802.1D STP Port State Transition Sequence
Estimated Time:1m 0s
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Topics

STP Port States and PortFast Benefits

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