OvS users:
You must use a version of OvS superior or equal to 2.1 so it can handle group tables. If you previously downloaded a Mininet-VM, you could create a new VM using its disk, and then update the OvS version within Mininet. Perform the update within mininet; you'll have to run the following commands:
$ cd /home/mininet/mininet/util
$ ./install.sh -V 2.3.1
This script will try updating your packages, but this operation can fail. If it does, run the command yourself then re-execute the script:
$ sudo apt-get update --fix-missing
Then rerun the install script. After a couple of minutes, the new version of OvS should be installed:
mininet@mininet-vm:~$ sudo ovs-vsctl show 1077578e-f495-46a1-a96b-441223e7cc22 ovs_version: "2.3.1"