'Create custom topo.'

def __init__(self):

    Topo.__init__(self)
    leftHost = self.addHost('h1')
    rightHost = self.addHost('h2')
    leftSwitch = self.addSwitch('s3')
    rightSwitch = self.addSwitch('s4')
    self.addLink(leftHost, leftSwitch)
    self.addLink(leftSwitch, rightSwitch)
    self.addLink(rightSwitch, rightHost)
