Refer to the exhibit.

The exhibit shows the advanced configuration of a VLAN where the Fail-safe option is checked. VLAN Fail-safe is a high-availability feature used to monitor network connectivity on a specific VLAN. When enabled, the BIG-IP system monitors the VLAN for network traffic. If the system does not detect any "useful" traffic on the VLAN within the specified Fail-safe Timeout (which is 90 seconds in the exhibit), it attempts to generate traffic by pinging the default gateway or other devices. If it still detects no traffic, the BIG-IP concludes that the VLAN is unreachable or the network interface has failed, and it triggers a "Fail-safe Action"—in this case, "Reboot" or a failover to the peer device in the HA group.

Because the administrator has just created a new VLAN that "does NOT yet have any traffic," the Fail-safe mechanism triggers immediately after the 90-second timeout period. Since no devices are yet communicating on this VLAN, the BIG-IP incorrectly assumes there is a hardware or cabling failure and forces a failover to ensure the standby device (which might have better connectivity) takes over. To prevent this unwanted failover, the administrator should uncheck the Fail-safe box for that specific VLAN until the VLAN is fully populated with active nodes and regular traffic. Once the application is live and traffic is flowing, Fail-safe can be re-enabled to provide an additional layer of redundancy. Auto Last Hop (Option A) and Source Check (Option B) are routing and security features that do not trigger HA failover events.