http://www.vmware.com/pdf/vsphere4/r40_u1/vsp_40_u1_availability.pdf

Network Isolation Addresses

A network isolation address is an IP address that is pinged to determine if a host is isolated from the network. This address is pinged only when a host has stopped receiving heartbeats from all other hosts in the cluster. If a host can ping its network isolation address, the host is not network isolated, and the other hosts in the cluster have failed. However, if the host cannot ping its isolation address, it is likely that the host has become isolated from the network and no failover action is taken.

By default, the network isolation address is the default gateway for the host. There is only one default gateway specified, regardless of how many service console networks have been defined, so you should use the das.isolationaddress[…] advanced attribute to add isolation addresses for additional networks. For example,  das.isolationAddress2 to add an isolation address for your second network, das.isolationAddress3 for the third, up to a maximum of das.isolationAddress9 for the ninth.

When you specify additional isolation address, VMware recommends that you increase the setting for the das.failuredetectiontime advanced attribute to 20000 milliseconds (20 seconds) or greater. A node that is isolated from the network needs time to release its virtual machine’s VMFS locks if the host isolation response is to fail over the virtual machines (not to leave them powered on.) This must happen before the other nodes declare the node as failed, so that they can power on the virtual machines, without getting an error that the virtual machines are still locked by the isolated node.

For more information on VMware HA advanced attributes, see “Customizing VMware HA Behavior,” on page 26.

das.isolationaddress
Sets the address to ping to determine if a host is isolated from the network. This address is pinged only when heartbeats are not received from any other host in the cluster. If not specified, the default gateway of the console network is used. This default gateway has to be a reliable address that is available, so that the host can determine if it is isolated from the network. You can specify multiple isolation addresses (up to 10) for the cluster: das.isolationaddressX, where X = 1-10. Typically you should specify one per service console. Specifying too many addresses makes isolation detection take too long and can affect VMware HA behavior.

das.usedefaultisolationaddress
By default, VMware HA uses the default gateway of the console network as an isolation address. This attribute specifies whether or not this default is used (true|false).