error is detected, the frame is already gone. Trying to deal with the problem is like locking the barn door after the horse has escaped.
解答：
存储-转发型交换机会对已经到达的帧进行检验，若发现帧有损坏，则将其丢弃；直通型交换机在一帧的目标头域进来时就开始转发，当发现传输帧有错误的时候，这个帧已经被传输出去了，这样就不能丢弃错误的帧了。
45．In Fig. 4-50 the switch in the legacy end domain on the right is a VLAN-aware switch. Would it be possible to use a legacy switch there? If so, how would that work? If not, why not?
Solution:
45. It would work. Frames entering the core domain would all be legacy frames,so it would be up to the first core switch to tag them. It could do this by using MAC addresses or IP addresses. Similarly, on the way out, that switch would have to untag outgoing frames.