Add tower topo.

+#!/usr/bin/env python
+from mininet.cli import CLI
+from mininet.net import Mininet
+from mininet.node import RemoteController, OVSKernelSwitch
+
+MAC = 12
+DPID = 16
+
+def string_to_hex(s, length):
+    """ Convert a string like 00:00 in to hex 0x0000 format"""
+    tmp = '{0:#x}'.format(int(s.replace(':', '').lstrip('0'),length))
+    return tmp
+
+def hex_to_string(h, length):
+    """Convert a hex number from 0x0000 to 00:00 format"""
+    tmp = h.lstrip('0x').zfill(length)
+    tmp = ':'.join(a+b for a,b in zip(tmp[::2], tmp[1::2]))
+    return tmp
+
+class Tower(object):
+    """ Create a tower topology from semi-scratch in Mininet """
+
+    def __init__(self, cname='flare', cip='15.255.126.183', k=4, h=6, 
+                 proto=None):
+        """Create tower topology for mininet
+            cname: controller name
+            cip: controller ip
+            k: number of leaf switches
+            h: number of hosts perl leaf switch
+        """
+
+        # We are creating the controller with local-loopback on purpose to avoid
+        # having the switches connect immediately. Instead, we'll set controller
+        # explicitly for each switch after configuring it as we want.
+        self.flare = RemoteController(cname, '127.0.0.1', 6633)
+        self.net = Mininet(controller=self.flare, switch = OVSKernelSwitch, 
+                           build=False)
+
+        self.cip = cip
+        self.spines = []
+        self.leaves = []
+        self.hosts = []
+        self.proto = proto
+
+        # Create the two spine switches
+        self.spines.append(self.net.addSwitch('s1'))
+        self.spines.append(self.net.addSwitch('s2'))
+
+        # Create two links between the spine switches
+        self.net.addLink(self.spines[0], self.spines[1])
+        self.net.addLink(self.spines[1], self.spines[0])
+        
+        # Now create the leaf switches, their hosts and connect them together
+        i = 1
+        c = 0
+        while i <= k:
+            self.leaves.append(self.net.addSwitch('s1%d' % i))
+            for spine in self.spines:
+                self.net.addLink(self.leaves[i-1], spine)
+
+            j = 1
+            while j <= h:
+                self.hosts.append(self.net.addHost('h%d%d' % (i, j)))
+                self.net.addLink(self.hosts[c], self.leaves[i-1])
+                j+=1
+                c+=1
+
+            i+=1
+
+    def run(self):
+        """ Runs the created network topology and launches mininet cli"""
+        self.run_silent()
+        CLI(self.net)
+        self.net.stop()
+
+    def run_silent(self):
+        """ Runs silently - for unit testing """
+        self.net.build()
+
+        # Start the switches, configure them with desired protocols and only 
+        # then set the controller
+        for sw in self.spines:
+            sw.start([self.flare])
+            if self.proto:
+                sw.cmd('ovs-vsctl set bridge %(sw)s protocols=%(proto)s' % \
+                           { 'sw': sw.name, 'proto': self.proto})
+            sw.cmdPrint('ovs-vsctl set-controller %(sw)s tcp:%(ctl)s:6633' % \
+                            {'sw': sw.name, 'ctl': self.cip})
+
+        for sw in self.leaves:
+            sw.start([self.flare])
+            sw.cmdPrint('ovs-vsctl set-controller %(sw)s tcp:%(ctl)s:6633' % \
+                            {'sw': sw.name, 'ctl': self.cip})
+
+    def pingAll(self):
+        """ PingAll to create flows - for unit testing """
+        self.net.pingAll()
+
+    def stop(self):
+        "Stops the topology. You should call this after run_silent"
+        self.net.stop()