in src/main/java/org/apache/cassandra/distributed/shared/AbstractBuilder.java [281:288]
public B withRacks(int dcCount, int racksPerDC, int nodesPerRack)
{
assert dcCount > 0 && racksPerDC > 0 && nodesPerRack > 0 : "dcCount, racksPerDC and nodesPerRack must be > 0";
for (int dc = 1; dc <= dcCount; dc++)
for (int rack = 1; rack <= racksPerDC; rack++)
withRack(dcName(dc), rackName(rack), nodesPerRack);
return (B) this;
}