Completed the A, B, C questions, using mininet script generating a fat tree topology.
Ubuntu 14.04 with mininet and ryu controller.
$ git clone https://github.com/pandadao/ECMP-mininet.git
$ cd ECMP-mininet
$ ryu-manager --observe-link ryu.app.simple_switch_13
After you open the ryu controller, you can see the following information like the picture:
Open a newterminal and type the following instruction:
$ sudo python my-fattree.py
And you will see the mininet create fat tree with k=4 topology information like the pictures: