Solution for "Exception: Could not find a default controller for switch ovsk"

[SummerXXXX]

In case you run command "sudo mn --custom ./topology.py --topo tutorialTopology --switch ovsk"
and get an error: "Exception: could not find a default controller for switch ovsk"

execute following commands could solve the problem:

sudo apt-get install openvswitch-testcontroller
sudo ln /usr/bin/ovs-testcontroller /usr/bin/controller 

source: https://stackoverflow.com/questions/17341076/mininet-cannot-find-required-executable-controller