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git clone http://github.com/mininet/mininet
mininet/util/install.sh -nfvnv
You can also download a pre-built Mininet VM.
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It should show a single loopback interface and no other interfaces. If this command doesn't work as expected, then Mininet will not work. If you have installed Mininet using apt-get, or from source on Ubuntu, Mininet should work correctly. However, you should make sure you don't have multiple or older versions of Mininet installed which might not work correctly with onos.py.
Once you've verified that mnexec works, you can try a sanity check for mn:
sudo mn --test pingallThis should complete without errors.