What is beacon probing?
Beacon probing is a network failover detection mechanism that sends out and listens for beacon probes on all NICs in the team and uses this information along with link status to determine link failure. Beacon probing detects failures, such as cable pulls and physical switch power failures on the immediate physical switch and also on the downstream switches.
How does beacon probing work?
ESX periodically broadcasts beacon packets from all uplinks in a team. The physical switch is expected to forward all packets to other ports on the same broadcast domain. Therefore, a team member is expected to see beacon packets from other team members. If an uplink fails to receive three consecutive beacon packets, it is marked as bad. The failure can be due to the immediate link or a downstream link.
Beaconing is most useful with three or more uplinks in a team because ESX can detect failures of a single uplink. When there are only two NICs in service and one of them loses connectivity, it is unclear which NIC needs to be taken out of service because both do not receive beacons and as a result all packets sent to both uplinks. Using at least three NICs in such a team allows for n-2 failures where n is the number of NICs in the team before reaching an ambiguous situation.
(more details you can see KB from VMware