Examkin

Question

Difficulty: EasySTP Port States and PortFast Benefits

When a non-PortFast interface on a Cisco switch using classic IEEE 802.1D Spanning Tree Protocol (STP) comes up, it undergoes a specific sequence of port states to prevent Layer 2 loops before it begins processing user traffic. Place the following STP port states in the correct chronological order from link initialization to full data forwarding.

  1. 1Blocking State: Discards data frames and does not learn MAC addresses while receiving BPDUs.
  2. 2Listening State: Transmits and receives BPDUs to elect root bridge and port roles, but discards data frames without learning MAC addresses.
  3. 3Learning State: Prepares to forward traffic by populating the MAC address table, but still discards user data frames.
  4. 4Forwarding State: Actively sends and receives data frames, learns MAC addresses, and processes BPDUs.

Answer

The correct operational sequence of classic 802.1D STP port states is Blocking State, followed by Listening State, then Learning State, and finally Forwarding State.
Under standard IEEE 802.1D Spanning Tree Protocol operational rules, an interface transitions sequentially from Blocking (preventing loops) to Listening (participating in BPDU role election), then to Learning (building MAC table entries), and finally reaches Forwarding (processing and forwarding user traffic).

Step-by-Step Solution

1
Identify the initial port state upon link initialization.
The switch interface starts in the Blocking state where it drops user frames and does not learn MAC addresses to prevent loops.
STP defaults to a loop-preventative state whenever a link transitions to up.
2
Determine the state focused on root bridge election and port role determination.
The interface moves to the Listening state, where BPDUs are processed and sent, but user data is still dropped and MAC learning is disabled.
The switch must determine topology roles (Root Port, Designated Port, or Alternate Port) before modifying table structures.
3
Determine the intermediate state focused on MAC table population.
After the first 15-second Forward Delay timer, the interface enters the Learning state, where it populates the MAC table from incoming frame headers but still drops user payload data.
Populating MAC addresses in advance prevents massive unicast flooding when the port starts forwarding.
4
Identify the final operational state allowing full traffic transmission.
After the second 15-second Forward Delay timer, the interface transitions to the Forwarding state.
The port is now confirmed loop-free and fully prepared to forward data frames.

Key Concept

IEEE 802.1D STP Port State Convergence Sequence
Estimated Time:45s
Rate this question

Topics

STP Port States and PortFast Benefits

More questions from this topic