The master ESXi server sends ICMP pings to the slave hosts over the management network. The management network is a port group that is configured for management traffic.

Also, by default, the default gateway is used as an isolation address. When the slave doesn't receive any network heartbeats from the master, it will try to reach its isolation address. If the isolation address is not reachable, the slave host will be considered isolated—meaning that access to the network is limited; based on your preferences, you can choose what to do with the VMs by configuring Response for Host Isolation:

You can change the isolation address from the default gateway to any other IP address based on your network design by using the advanced das.isolationaddress[0-9] cluster parameter. There can be up to 10 different isolation address.

It is also recommended to have a physical network that's highly available, so always use dual physical NICs to carry your management traffic as well as two physical switches to provide maximum network availability: