看到这本书的第五章link aggergation，在用mininet自建topo时，总是出现一下错误：

Traceback (most recent call last):

  File "<stdin>", line 1, in <module>

  File "build/bdist.linux-x86_64/egg/mininet/link.py", line 29, in <module>

  File "build/bdist.linux-x86_64/egg/mininet/node.py", line 67, in <module>

ImportError: cannot import name Link

后来在github的mininet主页找到了问题，

所以要先引进import node再import link，BINGO!