16.1.6 Auxiliary function for communication restoration

Uplink redundancy supports three features that assist communication recovery during switchover and switchback operations. Note that only one of the functions can be set for an uplink port.

• Functionality for sending and receiving flush control frames

  The Switch sends a flush control frame to upstream switches, where it is flooded, to clear their MAC address tables and restore communication. Upstream switches need to support the clearing of MAC address tables triggered by flush control frames.

• Functionality for updating MAC addresses

  The Switch sends an MAC address update frame to upstream switches to have them relearn the MAC addresses of terminals and restore communication. Upstream switches do not require the receiving functionality for this. However, the number of MAC addresses an upstream switch can relearn is limited. Note that it might take about 10 seconds for communication to be restored.

• Port resetting

  By temporarily bringing down the old active port, the host switch that detects a link-down clears MAC address entries learned on the port from MAC address table and recovers communication by flooding. The host switch does not require dedicated functionality, but the primary and secondary ports must be connected to the same host switch.

The flush control frame transmission/reception function assumes a device where the upper switch supports flush control frames, whereas MAC address update function and the port reset function assume a device that cannot receive flush control frames.