If the segment is attached to the root bridge, it has a cost of 0.For example, if the data has to travel over three 10 Mbps segments to reach the root bridge, the cost is 200 (100 + 100 + 0).

• 如果数据是根网桥产生的，那么花费就是0，如果经过了三个网段抵达根网桥的话，那么花费值就是200（100+100+0）。