new_contents: #!/usr/bin/python
  
  from mininet.topo import Topo
  
  from mininet.cli import CLI
  from mininet.net import Mininet
  from mininet.util import dumpNodeConnections
  from mininet.log import setLogLevel
  
  from mininet.node import RemoteController
  
  REMOTE_CONTROLLER_IP = "127.0.0.1"
  
  
  def simpleTest():
      # Create and test a simple network
      topo = SingleLoopTopo()
  
      net = Mininet(topo=topo,
                    controller=None,
                    autoStaticArp=True)
      net.addController("c0",
                        controller=RemoteController,
                        ip=REMOTE_CONTROLLER_IP,
                        port=6633)
      net.start()
      print "Dumping host connections"
      dumpNodeConnections(net.hosts)
      print "Testing network connectivity"
      net.pingAll()
      net.stop()
  
  
  class SingleLoopTopo(Topo):
      # Single switch connected to n hosts
      def __init__(self, **opts):
          # Initialize topology and default optioe
          Topo.__init__(self, **opts)
          switches = []
          hosts = []
  
          # create switches
          for s in range(3):
              switches.append(self.addSwitch('s%s' % (s + 1), protocols='OpenFlow13'))
  
          # create hosts
          for h in range(4):
              hosts.append(self.addHost('h%s' % (h + 1)))
  
          self.addLink(hosts[0], switches[1])
          self.addLink(hosts[1], switches[1])
          self.addLink(hosts[2], switches[2])
          self.addLink(hosts[3], switches[2])
  
          self.addLink(switches[0], switches[1])
          self.addLink(switches[0], switches[2])
          self.addLink(switches[1], switches[2])
  
  
  if __name__ == '__main__':
      # Tell mininet to print useful information
      setLogLevel('info')
      simpleTest()
      topo = SingleLoopTopo()
      net = Mininet(topo=topo,
                    controller=None,
                    autoStaticArp=True)
      net.addController("c0",
                        controller=RemoteController,
                        ip=REMOTE_CONTROLLER_IP,
                        port=6633)
      net.start()
      CLI(net)
      net.stop()
  
subject: add simple loop topology