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Difficulty: HardSpanning Tree Protocol Operations

When an active link fails in a legacy IEEE 802.1D Spanning Tree Protocol (STP) network, switches must complete a specific message exchange sequence to propagate the topology change and flush invalid MAC addresses. In what chronological sequence do these protocol actions occur following the link failure on a non-root switch?

  1. 1The switch experiencing the link failure generates a Topology Change Notification (TCN) BPDU.
  2. 2The detecting switch transmits the TCN BPDU out its Root Port towards the upstream switch.
  3. 3The upstream designated switch acknowledges receipt by returning a configuration BPDU with the Topology Change Acknowledgment (TCA) bit set.
  4. 4The Root Bridge receives the TCN BPDU and sets the Topology Change (TC) flag in its outbound configuration BPDUs.
  5. 5Non-root switches receiving the TC-flagged configuration BPDU temporarily shorten their MAC address table aging timer to the Forward Delay duration.

Answer

The correct chronological sequence for IEEE 802.1D topology change processing is: (1) The switch experiencing link failure generates a TCN BPDU, (2) The TCN BPDU is transmitted upstream out the Root Port, (3) The upstream switch acknowledges receipt using the TCA bit, (4) The Root Bridge receives the TCN and broadcasts configuration BPDUs with the TC flag set, and (5) Non-root switches temporarily shorten their MAC table aging timer to the Forward Delay duration.
In standard IEEE 802.1D STP, topology change handling follows a strict five-step progression: the local switch detects the failure and creates a TCN BPDU, sends it upstream through its Root Port, receives a hop-by-hop TCA acknowledgment from the next switch, the Root Bridge receives the TCN and broadcasts BPDUs with the TC bit set, and finally, network switches receive the TC flag and reduce their MAC address table aging timer to 15 seconds (Forward Delay).

Step-by-Step Solution

1
Detect event and create notification packet
The local switch detects link failure on a forwarding port and originates a TCN BPDU
Port state transitions trigger the local switch to initiate the STP topology change mechanism.
2
Propagate notification toward the root
The TCN BPDU is sent out the switch's Root Port upstream
TCN BPDUs must travel up the spanning-tree hierarchy toward the Root Bridge.
3
Perform hop-by-hop acknowledgement
The upstream switch sets the TCA bit in its next configuration BPDU back to the sender
TCA flag confirms receipt of the TCN BPDU so the sender ceases sending periodic TCN BPDUs every Hello interval.
4
Initiate domain-wide topology change alert
The Root Bridge receives the TCN BPDU and sets the TC flag in all outbound configuration BPDUs
The Root Bridge is the sole authority for propagating TC flags to all switches across the layer 2 network.
5
Accelerate MAC address table flushing
Non-root switches shorten their MAC table aging timer from 300 seconds to 15 seconds (Forward Delay)
Lowering the aging timer allows obsolete bridging table entries to age out rapidly without abruptly emptying the entire MAC table.

Key Concept

802.1D Topology Change Notification (TCN) Sequence
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Topics

Spanning Tree Protocol Operations

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