Skip to content

Commit

Permalink
Lan exercises
Browse files Browse the repository at this point in the history
  • Loading branch information
@obonaventure
obonaventure committed Dec 9, 2019
1 parent ebbf986 commit 71696c6
Showing 1 changed file with 311 additions and 0 deletions.
311 changes: 311 additions & 0 deletions exercises/lan.rst
View file
Original file line number Diff line number Diff line change
@@ -0,0 +1,311 @@
.. Copyright |copy| 2013,2019 by Justin Vellemans, Florentin Rochet, David Lebrun, Juan Antonio Cordero, Olivier Bonaventure
.. This file is licensed under a `creative commons licence <http://creativecommons.org/licenses/by/3.0/>`_
Local Area Networks: The Spanning Tree Protocol and Virtual LANs
=================================================================


.. inginious:: stp-bpdu


.. inginious:: stp-ports-state

.. inginious:: q-stp-1

Exercises
---------

1. Consider the switched network shown in Fig. 1. What is the spanning tree that will be computed by 802.1d in this network assuming that all links have a unit cost ? Indicate the state of each port.

.. figure:: /exercises/figures/ex-stp-switches.png
:align: center
:scale: 100

Fig. 1. A small network composed of Ethernet switches

2. Consider the switched network shown in Fig. 1. In this network, assume that the LAN between switches S3 and S12 fails. How should the switches update their port/address tables after the link failure ?


3. Consider the switched network shown in the figure below. Compute the Spanning Tree of this network.

.. tikz::
:libs: shapes, positioning, matrix, arrows

\tikzstyle{arrow} = [thick,->,>=stealth]
\tikzset{switch/.style = {diamond, draw, text centered, minimum height=2em, node distance= 2cm}, }
\tikzset{router/.style = {rectangle, draw, text centered, minimum height=2em}, }
\tikzset{host/.style = {circle, draw, text centered, minimum height=2em}, }
\tikzset{ftable/.style={rectangle, dashed, draw} }
\node[switch] (S3) {S5};
\node[switch, left of=S3] (S6) {S9};
\node[switch, right of=S3] (S7) {S10};
\node[switch, above of=S3] (S4) {S2};
\node[switch, below of=S3] (S9) {S4};

\path[draw,thick]
(S3) edge (S6)
(S3) edge (S7)
(S6) edge (S4)
(S4) edge (S7)
(S3) edge (S9)
(S9) edge (S7)
(S3) edge (S7);

4. Many enterprise networks are organized with a set of backbone devices interconnected by using a full mesh of links as shown in Fig.2. In this network, what are the benefits and drawbacks of using Ethernet switches and IP routers running OSPF ?

.. figure:: /exercises/figures/ex-stp-backbone.png
:align: center
:scale: 100

Fig. 2. A typical enterprise backbone network

5. In the network depicted in Fig. 3, the host `H0` performs a traceroute toward its peer `H1` (designated by its name) through a network composed of switches and routers. Explain precisely the frames, packets, and segments exchanged since the network was turned on. You may assign addresses if you need to.

.. figure:: /exercises/figures/ex-stp-switches_vs_routers.png
:align: center
:scale: 100

Fig. 3. Host `H0` performs a traceroute towards its peer `H1` through a network composed of switches and routers

6. In the network represented in Fig. 4, can the host `H0` communicate with `H1` and vice-versa? Explain. Add whatever you need in the network to allow them to communicate.

.. figure:: /exercises/figures/ex-stp-routing_across_VLANs.png
:align: center
:scale: 100

Fig. 4. Can `H0` and `H1` communicate ?

7. Consider the network depicted in Fig. 5. Both of the hosts `H0` and `H1` have two interfaces: one connected to the switch `S0` and the other one to the switch `S1`. Will the link between `S0` and `S1` ever be used? If so, under which assumptions? Provide a comprehensive answer.

.. figure:: /exercises/figures/ex-stp-switches_wo_STP.png
:align: center
:scale: 100

Fig. 5. Will the link between `S0` and `S1` ever be used?

8. Most commercial Ethernet switches are able to run the Spanning tree protocol independently on each VLAN. What are the benefits of using per-VLAN spanning trees ?

..
9. Consider the network shown below and assume that all routers use a link-state routing protocol.
.. figure:: ../../book/network/svg/ex-five-routers-redundant.png
:align: center
:scale: 100
Simple network with redundant links

a. Show the messages used by the routers to discover their neighbors and establish adjacencies.

b. Show the messages that propagate the adjacencies of A during the flooding phase.

c. For each router compute its routing table once the flooding is over.

d. Consider that link `B-C` fails and that router `B` is the first to detect the failure. Router `B` will flood its updated link state packet through the entire network and all routers will recompute their forwarding table. Compute the successive updates to the routers RIB, assuming that router `C` receives the updated link-state packet from router B before detecting the failure himself.

e. What would change if routers had used a distance-vector protocol instead.


Testing the Spanning Tree with IPMininet
----------------------------------------

IPMininet_ can also be used to configure the Spanning Tree protocol on Linux hosts that act as Ethernet switches. Let us consider the simple Ethernet network shown in the figure below.

.. tikz:: A simple Ethernet network
:libs: shapes, positioning, matrix, arrows

\tikzset{switch/.style = {diamond, draw, text centered, minimum height=2em, node distance= 2cm}, }

\node[switch] (S9) {S9};
\node[switch, left of=S9] (S6) {S6};
\node[switch, right of=S9] (S7) {S7};
\node[switch, above of=S9] (S4) {S4};
\node[switch, below of=S9] (S3) {S3};

\path[draw,thick]
(S3) edge (S6)
(S3) edge (S7)
(S6) edge (S4)
(S4) edge (S7)
(S3) edge (S9)
(S9) edge (S7)
(S3) edge (S7);


This network can be launched with the IPMininet_ script shown below. The entire
script is available from :download:`/exercises/ipmininet_scripts/stp.py`.

.. code-block:: python
import shlex
from ipmininet.iptopo import IPTopo
from ipmininet.ipnet import IPNet
from ipmininet.cli import IPCLI
class MyTopology(IPTopo):
def build(self, *args, **kwargs):
# Switches with manually set STP priority
s3 = self.addSwitch("s3", prio=3, lo_addresses=["2001:1::4/64"])
s4 = self.addSwitch("s4", prio=4, lo_addresses=["2001:1::4/64"])
s6 = self.addSwitch("s6", prio=6, lo_addresses=["2001:1::6/64"])
s7 = self.addSwitch("s7", prio=7, lo_addresses=["2001:1::7/64"])
s9 = self.addSwitch("s9", prio=9, lo_addresses=["2001:1::9/64"])
# Hub
#hub1 = self.addHub("hub1")
# Links
self.addLink(s3, s9, stp_cost=1) # Cost changed for both interfaces
l37=self.addLink(s3, s7)
l37[s3].addParams(stp_cost=1) # cost changed for s3->s7
l37[s7].addParams(stp_cost=1) # cost changed for s7->s3
self.addLink(s9, s7) # default cost of 1
self.addLink(s6, s9)
self.addLink(s6, s4)
self.addLink(s7, s4)
super(MyTopology, self).build(*args, **kwargs)
def post_build(self, net):
for s in self.switches():
command="/usr/sbin/tcpdump -i any --immediate-mode -c 50 -w ./stp-"+s+"-trace.pcap stp"
p = net[s].popen(shlex.split(command))
super(MyTopology, self).post_build(net)
net = IPNet(topo=MyTopology())
try:
net.start()
IPCLI(net)
finally:
net.stop()
The ``addSwitch`` method creates an Ethernet switch. It assigns a random MAC address to each switch and we can configure it with a priority that is used in the high order bits of the switch identifier. We add one IP address to each switch so that we can connect to them on mininet_. In practice, IPMininet_ configures the :manpage:`brtcl(8)` software that implements the Spanning Tree protocol on Linux. We can then create the links, configure their cost if required and launch tcpdump_ to capture the Ethernet frames that contain the messages of the Spanning Tree protocol.

The network contains five nodes and six links.

.. code-block:: python
mininet> nodes
available nodes are:
s3 s4 s6 s7 s9
mininet> links
s3-eth2<->s7-eth1 (OK OK)
s3-eth1<->s9-eth1 (OK OK)
s6-eth2<->s4-eth1 (OK OK)
s6-eth1<->s9-eth3 (OK OK)
s7-eth3<->s4-eth2 (OK OK)
s9-eth2<->s7-eth2 (OK OK)
By using :manpage:`brtcl(8)`, we can easily observe the state of the Spanning Tree protocol on the different switches. Let us start with ``s3``, i.e. the root of the Spanning Tree.

.. code-block:: console
mininet> s3 brctl showstp s3
s3
bridge id 0003.f63545ab5f79
designated root 0003.f63545ab5f79
root port 0 path cost 0
max age 20.00 bridge max age 20.00
hello time 2.00 bridge hello time 2.00
forward delay 15.00 bridge forward delay 15.00
ageing time 300.00
hello timer 1.03 tcn timer 0.00
topology change timer 0.00 gc timer 77.90
flags
s3-eth1 (1)
port id 8001 state forwarding
designated root 0003.f63545ab5f79 path cost 1
designated bridge 0003.f63545ab5f79 message age timer 0.00
designated port 8001 forward delay timer 0.00
designated cost 0 hold timer 0.02
flags
s3-eth2 (2)
port id 8002 state forwarding
designated root 0003.f63545ab5f79 path cost 1
designated bridge 0003.f63545ab5f79 message age timer 0.00
designated port 8002 forward delay timer 0.00
designated cost 0 hold timer 0.02
flags
The first part of the output of the :manpage:`brctl(8)` command shows the state of the Spanning Tree software on the switch. The identifier of this switch is ``0003.f63545ab5f79`` and the root switch is itself. There is no root port on this switch since it is the root. The path cost is the cost of the path to reach the root switch, i.e. 0 on the root. Then the switch reports the different timers.

The second part of the output provides the state of each switch port. Port ``s3-eth1`` is active and forwards data frames (state is set to `forwarding`). This port is a `designated` port. The cost of ``1`` is the cost associated to this interface. The same information is found for port ``s3-eth2``.

The state of switch ``s9`` is different. The output of :manpage:`brctl(8)` indicates that the root identifier is ``0003.f63545ab5f79`` which is at a distance of ``1`` from switch ``s9``. The root port on ``s9`` is port `1`, i.e. ``s9-eth1``. Two of the ports of this switch forward data packets, the root port and the ``s9-eth3`` which is a designated port. The ``s9-eth2`` port is a blocked port.

.. code-block:: console
mininet> s9 brctl showstp s9
s9
bridge id 0009.7ecc45e18e5b
designated root 0003.f63545ab5f79
root port 1 path cost 1
max age 20.00 bridge max age 20.00
hello time 2.00 bridge hello time 2.00
forward delay 15.00 bridge forward delay 15.00
ageing time 300.00
hello timer 0.00 tcn timer 0.00
topology change timer 0.00 gc timer 167.22
flags
s9-eth1 (1)
port id 8001 state forwarding
designated root 0003.f63545ab5f79 path cost 1
designated bridge 0003.f63545ab5f79 message age timer 20.00
designated port 8001 forward delay timer 0.00
designated cost 0 hold timer 0.00
flags
s9-eth2 (2)
port id 8002 state blocking
designated root 0003.f63545ab5f79 path cost 1
designated bridge 0007.2a6f5ef34984 message age timer 19.98
designated port 8002 forward delay timer 0.00
designated cost 1 hold timer 0.00
flags
s9-eth3 (3)
port id 8003 state forwarding
designated root 0003.f63545ab5f79 path cost 1
designated bridge 0009.7ecc45e18e5b message age timer 0.00
designated port 8003 forward delay timer 0.00
designated cost 1 hold timer 0.97
flags
:manpage:`brctl(8)` also maintains a MAC address table that contains the Ethernet addresses that have been learned on each switch port.

.. code-block:: console
mininet> s9 brctl showmacs s9
port no mac addr is local? ageing timer
1 2a:6f:5e:f3:49:84 no 257.92
1 62:60:d3:46:2f:12 no 257.92
3 7e:cc:45:e1:8e:5b yes 0.00
3 7e:cc:45:e1:8e:5b yes 0.00
2 a2:07:cb:02:90:4a yes 0.00
2 a2:07:cb:02:90:4a yes 0.00
1 d6:a1:b4:c8:de:72 yes 0.00
1 d6:a1:b4:c8:de:72 yes 0.00
1 f6:35:45:ab:5f:79 no 0.45
Thanks to the traces collected by tcpdump_, we can easily analyze the messages exchanged by the switches. Here is the fist message sent by switch ``s3``.

.. figure:: /exercises/figures/stp-packet1.png




.. include:: /links.rst

0 comments on commit 71696c6

Please sign in to comment.