Lab2: Create a simple network and use a POX controller to control the behaviors of switch


        In the following script, we will use one of the default pox controllers, i.e. forwarding.l2_learning.


# Copyright 2011 James McCauley


# This file is part of POX.


# POX is free software: you can redistribute it and/or modify

# it under the terms of the GNU General Public License as published by

# the Free Software Foundation, either version 3 of the License, or

# (at your option) any later version.


# POX is distributed in the hope that it will be useful,

# but WITHOUT ANY WARRANTY; without even the implied warranty of


# GNU General Public License for more details.


# You should have received a copy of the GNU General Public License

# along with POX.  If not, see <>.



An L2 learning switch.


It is derived from one written live for an SDN crash course.

It is somwhat similar to NOX's pyswitch in that it installs

exact-match rules for each flow.



from pox.core import core

import pox.openflow.libopenflow_01 as of

from pox.lib.util import dpid_to_str

from pox.lib.util import str_to_bool

import time


log = core.getLogger()


# We don't want to flood immediately when a switch connects.

# Can be overriden on commandline.

_flood_delay = 0


class LearningSwitch (object):


  The learning switch "brain" associated with a single OpenFlow switch.


  When we see a packet, we'd like to output it on a port which will

  eventually lead to the destination.  To accomplish this, we build a

  table that maps addresses to ports.


  We populate the table by observing traffic.  When we see a packet

  from some source coming from some port, we know that source is out

  that port.


  When we want to forward traffic, we look up the desintation in our

  table.  If we don't know the port, we simply send the message out

  all ports except the one it came in on.  (In the presence of loops,

  this is bad!).


  In short, our algorithm looks like this:


  For each packet from the switch:

  1) Use source address and switch port to update address/port table

  2) Is transparent = False and either Ethertype is LLDP or the packet's

     destination address is a Bridge Filtered address?


        2a) Drop packet -- don't forward link-local traffic (LLDP, 802.1x)


  3) Is destination multicast?


        3a) Flood the packet


  4) Port for destination address in our address/port table?


        4a) Flood the packet


  5) Is output port the same as input port?


        5a) Drop packet and similar ones for a while

  6) Install flow table entry in the switch so that this

     flow goes out the appopriate port

     6a) Send the packet out appropriate port


  def __init__ (self, connection, transparent):

    # Switch we'll be adding L2 learning switch capabilities to

    self.connection = connection

    self.transparent = transparent


    # Our table

    self.macToPort = {}


    # We want to hear PacketIn messages, so we listen

    # to the connection



    # We just use this to know when to log a helpful message

    self.hold_down_expired = _flood_delay == 0


    #log.debug("Initializing LearningSwitch, transparent=%s",

    #          str(self.transparent))


  def _handle_PacketIn (self, event):


    Handle packet in messages from the switch to implement above algorithm.



    packet = event.parsed


    def flood (message = None):

      """ Floods the packet """

      msg = of.ofp_packet_out()

      if time.time() - self.connection.connect_time >= _flood_delay:

        # Only flood if we've been connected for a little while...


        if self.hold_down_expired is False:

          # Oh yes it is!

          self.hold_down_expired = True

"%s: Flood hold-down expired -- flooding",



        if message is not None: log.debug(message)

        #log.debug("%i: flood %s -> %s", event.dpid,packet.src,packet.dst)

        # OFPP_FLOOD is optional; on some switches you may need to change

        # this to OFPP_ALL.

        msg.actions.append(of.ofp_action_output(port = of.OFPP_FLOOD))


        pass"Holding down flood for %s", dpid_to_str(event.dpid)) = event.ofp

      msg.in_port = event.port



    def drop (duration = None):


      Drops this packet and optionally installs a flow to continue

      dropping similar ones for a while


      if duration is not None:

        if not isinstance(duration, tuple):

          duration = (duration,duration)

        msg = of.ofp_flow_mod()

        msg.match = of.ofp_match.from_packet(packet)

        msg.idle_timeout = duration[0]

        msg.hard_timeout = duration[1]

        msg.buffer_id = event.ofp.buffer_id


      elif event.ofp.buffer_id is not None:

        msg = of.ofp_packet_out()

        msg.buffer_id = event.ofp.buffer_id

        msg.in_port = event.port



    self.macToPort[packet.src] = event.port # 1


    if not self.transparent: # 2

      if packet.type == packet.LLDP_TYPE or packet.dst.isBridgeFiltered():

        drop() # 2a



    if packet.dst.is_multicast:

      flood() # 3a


      if packet.dst not in self.macToPort: # 4

        flood("Port for %s unknown -- flooding" % (packet.dst,)) # 4a


        port = self.macToPort[packet.dst]

        if port == event.port: # 5

          # 5a

          log.warning("Same port for packet from %s -> %s on %s.%s.  Drop."

              % (packet.src, packet.dst, dpid_to_str(event.dpid), port))



        # 6

        log.debug("installing flow for %s.%i -> %s.%i" %

                  (packet.src, event.port, packet.dst, port))

        msg = of.ofp_flow_mod()

        msg.match = of.ofp_match.from_packet(packet, event.port)

        msg.idle_timeout = 10

        msg.hard_timeout = 30

        msg.actions.append(of.ofp_action_output(port = port)) = event.ofp # 6a




class l2_learning (object):


  Waits for OpenFlow switches to connect and makes them learning switches.


  def __init__ (self, transparent):


    self.transparent = transparent


  def _handle_ConnectionUp (self, event):

    log.debug("Connection %s" % (event.connection,))

    LearningSwitch(event.connection, self.transparent)



def launch (transparent=False, hold_down=_flood_delay):


  Starts an L2 learning switch.



    global _flood_delay

    _flood_delay = int(str(hold_down), 10)

    assert _flood_delay >= 0


    raise RuntimeError("Expected hold-down to be a number")


  core.registerNew(l2_learning, str_to_bool(transparent))





from import Mininet

from mininet.node import Controller

from mininet.topo import SingleSwitchTopo

from mininet.log import setLogLevel


import os


class POXBridge( Controller ):

    "Custom Controller class to invoke POX forwarding.l2_learning"

    def start ( self ):

        "Start POX learning switch"

        self.pox = '%s/pox/' % os.environ[ 'HOME' ]

        self.cmd( self.pox, 'forwarding.l2_learning &' )

        #self.cmd( self.pox, 'forwarding.hub &' )

    def stop ( self ):

        "Stop POX"

      self.cmd( 'kill %' + self.pox )


controllers = { 'poxbridge': POXBridge }


if __name__ == '__main__':

    setLogLevel( 'info' )

    net = Mininet( topo=SingleSwitchTopo( 2), controller=POXBridge )



    h1, h2 = net.get( 'h1', 'h2' )

    #net.iperf((h1, h2))

    result1=h1.cmd('ping -c 5')

    print result1

    result2=h2.cmd('ping -c 5')

    print result2






Dr. Chih-Heng Ke (

Department of Computer Science and Information Engineering,

National Quemoy University, Kinmen, Taiwan.