16.1.7 Flash control frame transmission/reception function

(1) Transmit Operation

When the active port is changed due to a failure on the communication link or when an operation command is executed to change the active port, the Switch can send a flush control frame to request an upstream switch to clear its MAC address table. You can configure the sending of flush control frames for each uplink port pair, and you can specify the destination VLANs.

If there is any switch or router on the network whose MAC address table you do not wish to clear, configure a special VLAN for sending and receiving flush control frames. Then, set the configuration to send flush control frames only to the specified VLAN to limit the switches and routers that clear their MAC addresses when flush control frames are sent.

The Switch sends flush control frames from the new active port immediately after the port is enabled.

When you use a trunk port to send flush control frames, you need to specify the destination VLAN. For access ports, MAC ports, and protocol ports, the Switch sends untagged flush control frames regardless of whether the destination VLAN is specified.

(2) Reception operation

When the Switch receives a flush control frame, it clears its MAC address table.

You do not need to specify any configuration command to receive flush control frames. However, when the Switch is configured to send flush control frames to a specific VLAN, that VLAN must be enabled to receive flush control frames.

The following figure shows the difference in switchover operation according to the use of flush control frames.

Figure 16-4: Differences in switching operations due to use of flush control frames

Normal operation

The primary port on the Switch performs communication. Upstream switches learn the MAC address of the user terminal via the current communication path.

If the primary port fails (when flush control frames are not sent)

If the Switch is not configured to send flush control frames, although the secondary port is now active, upstream switch B retains the MAC address of the user terminal on the previous port. Therefore, communication is not restored until the MAC address learned by upstream switch B is erased or the user terminal sends traffic to upstream switch B.

If the primary port fails (when flush control frames are sent)

If the Switch is configured to send flush control frames, it sends a flush control frame to request that upstream switch B clear its MAC address table as soon as the secondary port becomes active. Therefore, communication can be restored immediately.