it will use the default gateway, and you can specify an additional address via: cluster settings -> ha -> advanced options
type das.isolationaddress and specify the additional ip address.
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I have a customer whose default gateway does not respond to pings.
So I increased das.failuredetectiontime, defined das.isolationaddress and das.isolationaddress2 to point to two reliable servers and disabled pings to the default gateway by setting das.usedefaultisolationaddress = false.