For the fourth programming project, you will be simulating a small data-center
network and control it by using the SDN paradigm. Here, using mininet and the
techniques you learned in the third project, you will created a network, design an ARP
responder, install the rules to make traffic flow and deal with link failures which are a
common occurrence in real life networks.
This project can be done in upto groups of three. You can keep your groups from
project three if you like, but do mention your groups in moodle.
Leaf-Spine architecture is widely adopted in modern data centers. Leaf-spine is a
two-layer network topology composed of leaf and spine switches. Hosts are connected
to leaf switches and leaf switches are connected to spine switches. Leaf switches
through the spine that form the access layer that delivers network connection points for
the hosts throughout the data center. Spine switches have high port density and
bandwidth available and form the core of the architecture.
For this project you will also use a leaf spine topology (which is given to you,
please see the topology.py file). The topology can also be seen in the figure below
Leaf Spine Topology
Here switches s4 and s5 are the spine switches, switches l1,l2 and l3 are the leaf
switches and h1-h6 are the hosts connected in this topology.
For sending traffic between the hosts, please follow the following rules:
● If both hosts are connected to the same leaf switch, then the traffic
between the hosts will only traverse through that leaf switch. For example,
the path between h1 and h2 is h1-l1-h2.
● If the two hosts are connected to different leaf switches, then the traffic
between them need to go through a spine switch. The policy of choosing
the spine switch is: traffic source from h1, h3 and h5 should traverse
through spine switch s4 and traffic source from h2, h4 and h6 should
traverse through spine switch s5. For example, the path between h1 and
h5 is h1-l1-s4-l3-h5, while the path between h2 and h5 is h2-l1-s5-l3-h5.
The project consists of three parts which are described as follows. The controller
program you need to implement should consist of these three parts.
Part 1 ARP Responder (30 points)
For traffic delivery, MAC addresses are used by the network, but each host on
the network initially does not know the other host’s MAC address. Here the ARP
responder comes into play. It keeps track of all the IP-to-MAC mapping and responds to
the ARP queries sent from the hosts. The mapping can be known beforehand and
stored in the controller for responding to the queries.
You need to implement the ARP responder as one component in your controller
program, and you can not use any built in mechanism in the controller for arp
SInce the arp responder resides at the controller, rules should be installed at the
switches to forward all ARP requests directly to the controller. The controller creates
ARP replies and then sends the ARP replies back to the switches. The switches further
deliver the ARP replies to the hosts.
For checking whether the arp responder is working correctly, use the command
Part 2 Installing Rules (30 points)
Since the topology and the link details are known beforehand, you can figure out
the rules that will be required for forwarding traffic in the network.In this project you will
need to install rules in the switches so that the switches can know where to forward
In this routing component, due to the static topology and the initial installation of
rules, after the topology has been bootstrapped, there should be negligible traffic
between the controller and the switches about traffic handling.
The leaf spine topology contains forwarding loops, flooding packets will cause a
broadcast storm and therefore it should be avoided at all costs. Make sure that the rules
are working properly.
For checking that the rules are working, make sure that all hosts are reachable
by other hosts. For example, use pingall.
Part 3 Link Failures (40 points)
In data centers, link failure is a common occurrence, due to the fact that there
exist a large number of links and the links are not 100% reliable. Mitigating link failures
then becomes one of the most important management tasks for operators. To do that,
one approach is to add a certain degree of redundancy to the data center architecture,
and design a proper mechanism to reroute the traffic through “good” links if link failure
happens. In this part, you will simulate link failures and implement mechanism to deal
For simulating the failure of a link, please use the “link down” command of
mininet in the CLI.
For completing this part you have to do the following two things:
1. Detect Link Failure:
You should be able to detect that a link has failed in the topology. You can
use built-in mechanism of the controller for this part.
2. Mitigating Link Failure:
After detection of failures, you should mitigate the effect of such failures by
installing rules at the appropriate switches to bypass the failed link so that end-to-end
In this part, you should fail all three links of any one spine switch and check
whether pingall works. Please note that only one spine switch’s links should be affected,
and all its links will be disconnected.
We assume that even in the event of link failure there is always a physical path
available between the hosts. Therefore if a subset of each spine switch links are failed
such that a physical path is available, even in that case your solution should route
traffic. For example if l1-s4, l2-s4 and l3-s5 are all failed even then traffic should flow.
● The above three parts are different components of your controller and have to
run side by side. So make sure that there is no error when all three components
are run concurrently.
● For obtaining the details about link connection on the controller, you can use the
fact that mininet assigns ports sequentially to switches, hence the details of links
will always be the same and the details can be seen in the mininet CLI using the
● The mac address that mininet assigns to the hosts are also sequential, so h1
gets the mac 00:00:00:00:00:01, h2 gets the mac 00:00:00:00:00:02 and so on.
Make sure to use the option “–mac” for this behaviour to exist.
● The IP address for the hosts are also sequentials and the details can be found
using the command “ifconfig”.
● For this project you will use the leaf spine topology that has been provided to
you. Please do not make any changes to the topology file.
For running the topology while using POX as the controller, use the
sudo mn –controller=remote –custom topology.py –topo mytopo –mac
If you are using Floodlight as the controller, then you the following command
sudo mn –controller=remote,ip=127.0.0.1,port=6653 –custom topology.py
–topo mytopo –mac
1) All the code files for controller.
2) Readme for describing the following things
a) Code implementation and design details
b) Any relevant details for running the project
c) A detailed breakdown of individual contributions
3) A session file to show the output for the following three things
a) The arp table of each host after the pingall (you can use the command h1
arp -n for the arp table of h1).
b) Pingall result with the initial topology
c) Pingall result with one spine switch ( s4) link failures (all three links should