TCN BPDUs

TCN BPDUs are used to inform other switches of port changes.

TCNs are injected into the network by a non-root switch and propagated to the root. Upon receipt of the TCN, the root switch will set a Topology Change flag in its normal BPDUs. This flag is propagated to all other switches to instruct them to rapidly age out their forwarding table entries.

Consider these configuration rules:

TCN BPDUs are sent per VLAN.
TCN BPDUs are sent only in those VLANs in which a topology change is detected.
TCN BPDUs are sent only in those VLANs for which the bridge is not the root bridge.
If a topology change is detected on a VLAN for which the bridge is the root bridge, the topology change flag is set in the configuration BPDU that is sent out.

For a given link, in conjunction with the configuration rules, a TCN BPDU is sent out as follows:

On an access port, only a standard IEEE TCN BPDU is sent out. This TCN BPDU corresponds to a topology change in the access VLAN.
On a trunk port, if VLAN 1 is allowed (either untagged or untagged), a standard IEEE TCN BPDU is sent for VLAN 1.
On a trunk port, if the native VLAN is not 1, an untagged TCN BPDU is sent to Cisco or Extreme proprietary MAC address for that VLAN.
On a trunk port, a tagged TCN BPDU is sent to Cisco or Extreme proprietary MAC address for a tagged VLAN.

As part of the response to TCN BPDUs, the Topology Change and Topology Change Acknowledgment flags are set in all configuration BPDUs corresponding to the VLAN for which the TCN was received.

When a topology change is detected on a trunk port, it is similar to detecting topology changes in each VLAN that is allowed on that trunk port. TCN BPDUs are sent for each VLAN as per the rules.