OSPF (Open Shortest Path First) Case Study: Anil Nembang

Anil Nembang-C0478BSBS1013

Data Communication, Network and Digital
Communication
Student Name: Anil Nembang
Student ID: C0478BSBS1013
Lecturer: Mr. Nigel Kermode
Cardiff Metropolitan University

Questions
1. In TCP/IP, there are 2 main types of Interior Routing Protocols (IRPs) namely distance vector
routing protocols, such as RIP, and link state routing protocols, such as OSPF and IS-IS.
Write a short report (no more than 750 words) that explains each of the 6 terms in bold and
why each is significant in the context of a data network.
2. Compare and contrast the role of areas in the 2 main link state routing protocols, namely
OSPF and IS-IS. You should briefly examine the role of virtual links in OSPF. Write your
answer in the form of a short report of no more than 750 words.
3. Consider the following case study which depicts a network for the London School of Routing;
it comprises a number of routers and PCs. The interior routing protocol for the entire
network is to be OSPF and the network manager has decided to partition the network into 4
areas as shown:

You are to develop an addressing scheme and apply the appropriate addresses/subnet
masks to the router interfaces and the 2 PCs. You should also specify the configuration of
the OSPF areas (paying particular attention to the requirement for a virtual link)
You are not required to use the Packet Tracer tool but you may wish to experiment with it.
A Packet Tracer file of the Central London Infrastructure of the London School of Routing is
included with this assignment to help you get started. If you do use Packet Tracer you might
wish to demonstrate that your combined addressing and routing scheme works by pinging
between the 2 PCs in both directions.
Whether you choose to use Packet Tracer or not, write a short report of no more than 1000
words explaining the logic behind your addressing scheme and your configuration of the
OSPF routing protocol for the Central London Infrastructure of the London School of Routing.

Table of Contents
1

TCP/IP Architectural Model: ........................................................................................................... 4

2

Interior Routing Protocol (IRP):....................................................................................................... 5
2.1

Distance Vector Routing Protocol: .......................................................................................... 6

2.1.1
3

Routing Information Protocol (RIP):................................................................................ 6

Link State Routing Protocol:............................................................................................................ 6
3.1
3.2

4

Open Shortest Path First (OSPF): ............................................................................................ 6
Intermediate System to Intermediate System (IS-IS): ............................................................ 7

Area: ................................................................................................................................................ 7
4.1

Importance of area in OSPF: ................................................................................................... 7

4.2

Importance of area in IS-IS:..................................................................................................... 8

4.2.1
4.3
5

IS-IS Levels: ...................................................................................................................... 8

Role of virtual link: .................................................................................................................. 9

Open Shortest Path First Configuration: ......................................................................................... 9
5.1

Addressing in Area 99 (OSPF Multiaccess network): ............................................................ 10

5.2

OSPF Configuration on the Network ..................................................................................... 11

6

PC-X pinging PC-Y: ......................................................................................................................... 13

7

Tracing route form PC-X to PC-Y: .................................................................................................. 14

8

Virtual Link Screenshot: ................................................................................................................ 15

9

Reference: ..................................................................................................................................... 16

1 TCP/IP Architectural Model:
The TCP/IP protocol suit is named for two of its most important protocols: Transmission Control
Protocol (TCP) and Internet Protocol (IP). A less used name for it is the Internet Protocol Suit, which
is the phrase used in Official Internet Standards Documents. The main design goal of TCP/IP was to
build an interconnection of networks, referred to as an internetwork or Internet, that provides
universal communication services over heterogeneous physical networks. The clear benefits of such
an internetwork is the enabling of communication between hosts in different networks, perhaps
separated by a large geographical area (Praziale L. et al, 13/12/2006).

Figure 1: Internet examples: Two interconnected sets of networks, each seen as one logical network

Another important aspect of TCP/IP internetworking is the creation of a standardize abstraction of
the communication mechanisms provided by each type of network. Each physical network has its
own technology-dependent communication interface, in the form of programming interface that
provides basic communication functions. TCP/IP provides communication services that run between
the programming interfaces of a physical network and user application. TCP/IP protocols are
modelled in four layers (Praziale L. et al, 13/12/2006).


Figure 2: The TCP/IP protocol stack : Each layer represents a package of functions (source)

Figure 3: Detailed architecture model with example (Source)

2 Interior Routing Protocol (IRP):
Routing is the process of moving packets across a network from one host to another. It is usually
done dedicated device called router.
Interior Routing Protocol is also known as Interior Gateway Protocol (IGP). IRP passes information
between routers within Autonomous System (AS). Autonomous System is the unit of router policy,
either single network or group of networks that is controlled by a common network administrator
(or a group of administrators) on behalf of single administrative entity such as business enterprise,
university, business division etc. Networks within and autonomous system communicate routing
information to each other using an Interior Gateway Protocol (IGP). An autonomous system shares
routing information to other autonomous system using Border Gateway Protocol (BGP). The routing
information can also be used by the internet protocol (IP) or other network protocols to specify how
to route transmissions (techtarget.com, 2013).

2.1 Distance Vector Routing Protocol:
A routing protocol is a set of rules used by routers to determine the most appropriate paths into
which they should forward packets towards their intended destinations. A packet is most
fundamental unit of data transmission on the internet or other TCP/IP networks.
Distance Vector Routing Protocol is a simple routing protocol used in packet switched networks that
utilizes distance to decide the best packet forwarding path. Distance is typically represented the hop
count. A hop is the trip that a packet takes from one router to another as it traverse a network on
the way to its destinations.
Distance Vector Routing Protocols are simple, require little management, and are efficient for small
networks. However, they have poor convergence properties and do not scale well. Convergence is
the process of routers updating their routing tables (i.e. built in database) and agreeing with each
other on optimal routes for forwarding packets (Linfo.org, 2012).
2.1.1 Routing Information Protocol (RIP):
Routing Information Protocol (RIP) is a standard-based, distance vector, Interior Routing Protocol
used by routers to exchange routing information. RIP uses hop count to determine the best location
between two paths. Hop count is the number of routers that the packet need to traverse to reach
the destination network. The maximum number of allowable hops a packet can traverse in an IP
network implementing RIP is 15.
In RIP network, each router broadcasts its entire RIP table to its neighbouring routers every 30
seconds. When a router receives neighbour RIP tables, it updates its own routing table and send the
updates to neighbour tables.

3 Link State Routing Protocol:
Link State Routing is complex routing technique in which each routers shares information with other
routers about the reach ability of other networks and the metrics (metric is the measurement of
performance in product or system like program or network) to reach the other networks in order to
determine the best path. The metric is based on hops, link speeds, traffic congestions and other
factors as determined by network designers.
In link state routing, every router on the network receives the map of the connectivity of the
network in the form of graph showing which nodes (computers, network devices, routers, switches)
are connected to which other nodes. Each router then independently calculates the best next hop
for every possible destination in the network. The collection of best next hops forms routing table of
a router. Link state routers use Dijkstra's algorithm to calculate the lowest cost path invented by
Dutch computer scientist Edsger Dijkstra.

3.1 Open Shortest Path First (OSPF):
OSPF (Open path first) is router protocol used in large autonomous system network that is used
installed in many of today's corporate network. OSPF is designated by the Internet Engineering
Taskforce (IETF) as one of several Interior Gateway Protocol (IGP).

Using OSPF a host that changes to a routing table or detects a change in a the network immediately
multicasts the information to all other hosts in the network so that all will have same routing
information. Unlike RIP where the entire routing table is sent in every 30 second, the hosts using
OSPF sends only the part that has changed and only when the change take place. Rather than
counting simple number of hosts, OSPF uses extra information consisting description of link state. In
OSPF user can also assign cost metric so that certain paths are given priority.

3.2 Intermediate System to Intermediate System (IS-IS):
Intermediate System to Intermediate System (IS-IS) protocol is an intra-domain Open System
Interconnection (ISO) dynamic routing protocol specified in International Organization for
Standardization. The protocol is designed to operate in OSI connectionless Network Service (CLNS).
A two level hierarchy is used to support large routing domains. A large domain may be
administratively divided into number of areas. Routing within an area is referred to as level 1
routing. Routing between two areas is called level 2 routing. Level 1 intermediate system keeps
track of the routing within in an area. Level 2 routing Intermediate System keeps track of the path to
destination areas. On broadcast multi-access media, a designated Intermediate System (DIS) is
elected and will conduct the flooding over the media. (Cisco.com, 2012)

4 Area:
An area is a logical connection of networks, routers, and links that have the same identification.
Areas limit the scope of route information distribution. A router within an area must maintain a
topological database for the area to which it belongs. The router does not have detailed information
about a network topology outside the area and hence reducing the size of its database.





Every time the route flaps, it initiates shortest-path-first algorithm calculations on all routers
in that area. This cause high CPU utilization.
The size of routing table will be small
The link-state topology becomes more manageable.
Significantly reduces Link State Database (LSDB)

4.1 Importance of area in OSPF:
OSPF networks in an autonomous system are administratively grouped into areas. Within an area,
the topology database contains only information about the area, link-state advertisements are
flooded only to nodes within the area, and routes are computed only within the area. The topology
of a n area is hidden from the rest of the AS, thus significantly reducing routing traffic in the AS.
OSPF has only two level of hierarchy. One backbone are and all other stub areas attached to
backbone area. Areas are used to group routers into manageable groups that exchange routing
information locally, but summarizes that routing information when adverting the routes externally.
Area Boarder Router (ABR) are used to connect the areas. Each area will elect a Designated Router

(DR) and a backup designated router (BDR) to assist flooding Link State Advertisements (LSAs)
throughout the area.

4.2 Importance of area in IS-IS:
Two-level hierarchy is used to support large routing domains. A large domain may be
administratively divided into areas. Routing within an area is referred to as Level 1 routing. Routing
between area is referred to as Level 2 routing. A level 2 intermediate system keeps track of the path
to destination areas. A level 1 intermediate system keeps track in its own area. For a packet destined
for another area, a level 1 IS sends the packet to the nearest Level 2 IS in its own area, regardless of
what destination area is. Then the packet travels via Level 2 routing to the destination area.

Figure 4: L1, L2 and L1L2 routers in IS-IS (source)

4.2.1






IS-IS Levels:
Level-1 routers:
o has neighbours only on the same area.
o Has Level 1 LSDB (Link State Database) with all routing information for the area.
Level-2 routers:
o May have neighbours in the same or other areas
o Has level-2 LSDB with all routing information about inter-area.
Level-1-2 routers:
o May have neighbours on any areas.
o Has two separate LSDBs : level-1 LSDB and level-2 LSDB

IS-IS does not have a backbone area as such OSPF. Instead the backbone area of IS-IS is the
contiguous collection of Level-2 capable routers. ISIS area borders are links not the routers which
makes IS-IS more flexible and scalable. (Smith P.,2009).

4.3 Role of virtual link:
In large networks with many areas, in which direct connectivity between all areas and the
backbone area is physically difficult or impossible. So, virtual link is configured to connect
non-contiguous non-backbone area to backbone area. Virtual links are also used to establish
link among non-contiguous backbone areas.
link acts as a tunnel which forwards LSAs to the backbone area via second intermediate area called
transit area.

Figure 5: Virtual Link (source)

5 Open Shortest Path First Configuration:
In all OSPF network areas except OSPF Area 99, the routers have a point to point connection. Since
these are point to point connections, the given address range can be sub netted into a /30 range
which will give two usable addresses for the two connecting interfaces between devices. e.g. The
OSPF network Area 120 has four point to point connection between the router serial interfaces (s)
and one point to point Ethernet (e) connection to the PC. The address range of 11.22.33.0 /24 has
been assigned to that area so this means that since there are 5 point to point connections we can
subnet the range into 5 /30 subnets. Each range having four addresses. The usable addresses will be
the second and third addresses of each range. The first address (Network) and the last address
(Broadcast) are not usable. Using Area 120 as an example:
11.22.33.0/30
Available address
11.22.33.0
11.22.33.1
11.22.33.2
11.22.33.3

PC X- Earls Court
Usability
Unusable (Network address)
Usable
usable
Unusable (Broadcast address)

Allocation
×
PC-X (f/0)
Earls court (f/0)
×

11.22.33.4 / 30
Available address
11.22.33.4
11.22.33.5
11.22.33.6
11.22.33.7

11.22.33.8/30
Available address
11.22.33.8
11.22.33.9
11.22.33.10
11.22.33.11

11.22.33.12 /30
Available address
11.22.33.12
11.22.33.13
11.22.33.14
11.22.33.15

11.22.33.8 /16
Available address
11.22.33.16
11.22.33.17
11.22.33.18
11.22.33.19

Earls Court–Kensington
Usability
Usable
Usable

Allocation
×
Earls court (S0/0/0)
Kensington (S0/0/0)
×

Kensington-Westminster
Usability
Usable
Usable

Allocation
×
Kensington (s 0/0/1)
Westminster (s 0/0/1)
×

Westminster–Chelsea
Usability
Usable
Usable

Allocation
×
Westminster (s 0/0/0)
Chelsea (s 0/0/0)
×

Chelsea–Earls Court
Usability
Usable
Usable

Allocation
×
Chelsea (s 0/0/1)
Earls court (s 0/0/1)
×

/30 sub netting scheme gives two usable addresses and the number of IP addresses required in each
point to point connection is also two. Therefore the IP addresses will not be wasted. One of the
logics of CIDER is to optimally utilise the IP addresses.

5.1 Addressing in Area 99 (OSPF Multiaccess network):
This sub netting addressing scheme applies to all the other areas within the Central London
Infrastructure except Area 99 which is OSPF Multiaccess network. All three routers in this area
access the transit switch. Sub netting addressing scheme of /29 is applied in this area as we need 3 IP
addresses. /29 gives 8 addresses among which only 6 are usable. And 3 IP addresses among 6 are
allocated to interfaces of 3 routers in Area 99. (if /29 sub netting scheme is applied, fewer IP
addresses will be wasted in comparison to any other sub netting scheme in this context).
Available IP addresses
33.44.55.0

Usability
Unusable (Network Address)

Allocation
×

33.44.55.1
33.44.55.2
33.44.55.3
33.44.55.4
33.44.55.5
33.44.55.6
33.44.55.7

Usable
Usable
Usable
Usable
Usable
Usable
Unusable (Broadcast Address)

London Bridge (f/0)
Peckham Rey (f/0)
Bermondsey (f/0)
Wasted
Wasted
Wasted
×

5.2 OSPF Configuration on the Network
Since the interior routing protocol will be OSPF and the network has been partitioned into four
areas, OSPF will have to firstly be enabled on all the routers within the network with the following
Router(config)#router ospf process-number
The process number is a number given to distinguish the OSPF from other processes on the router
Now that the Routers on the network have been enabled to run OSPF. The OSPF process will need to
know which networks are going to have their routes advertised and what areas they belong to. To
do this, the following command will be used
Router(config-router)#network address wildcard-mask area area-number
The network address will be the first address of the sub net IP range assigned to the interface wild
card mask of 4 groups of 8 bits. The 0 bit means no other network outside the range will be
advertised whereas the 1 bit means that any address with IP range can. i.e. 11.22.23.0 0.0.0.255
means any address outside 11.22.33.x won’t be advertised but any address within the .252 subnet
will.
So for the area 120, the earls court OSPF configuration will looks like this.
Earls Court(config)#router ospf 1
Earls Court(config)#network 11.22.33.12 0.0.0.255 area 120
Earls Court(config)#network 11.22.33.16 0.0.0.255 area 120
The routers that are between two areas, in this case Westminster, London Bridge and Bermondsey
are known as ABRs or Area Border Routers. As they advertise routes from different areas, they are
configured to show the network of all the area networks they have borders with as shown below
using the Westminster router
Westminster(config)#router ospf 2
Westminster(config)#network 11.22.33.0 0.0.0.255 area 120

The Area 99 has a 3560 multilayer switch connected to three routers. London Bridge, Bermondsey
and Peckham. The London Bridge router being the designated router which means it’s the central
point for all the incoming LSA’s while The Peckham Rye router is the Backup designated router, with

the Bermondsey router being the ABR router between Area 99 and Area 123. Since /29 sub netting is
done in Area 99, the subnet mask will be 255.255.255.248.
The Central London infrastructure has four areas within the autonomous system. One area should be
the backbone area while all the other areas must be connected to the backbone area in order to get
central routing information. In this case the Area 0 is the backbone router. Area 120 and 99 is
connected to Area 0. Area 123 is connected to Area 99 but not Area 0 as a result area 123 will not be
able to get central routing information. In order to overcome this, a virtual link is set up between the
Bermondsey router and the London bridge router. Area 99 will be known as the transit area where
the virtual link is to be set up.
The virtual link should be configured on both the London Bridge ABR router and the Bermondsey
ABR router. The configuration will look like this:
Bermondsey(config)#router ospf 2
Bermondsey(config)#network 44.55.66.0 0.0.0.255 area 123
Bermondsey(config)#network 33.44.55.0 0.0.0.255 area 99
Bermondsey(config)#area 99 virtual-link “London Bridge Router ID”
London Bridge(config)# router ospf 3
London Bridge(config)# network 33.44.55.0 0.0.0.255 area 99
London Bridge(config)# area 99 virtual-link “Bermondsey Router ID”
The router ID is usually the highest IP address on the router or the loopback address. The loopback
address is used for testing connection on the network. Or, any 32 bit IP address can be assigned as
Router ID with following command:
Router(config)#router ospf process-id
Router(config)#router-id (32-ip address)
In this project 2.2.2.2 and 1.1.1.1 are assigned to ARBs in London Bridge and Bermondsey
respectlively.
Ex: London Bridge (config)#router-id 2.2.2.2
Now that a virtual link has been created, all the routers on the network should now have all the
routes advertised on their respective routing tables.

6 PC-X pinging PC-Y:
This is the screenshot where PC-Y with IP address 44.55.66.6 is responding to the ping command of PC-X with IP address 11.22.33.1.

Figure 6: Ping form PC-X to PC-X

7 Tracing route form PC-X to PC-Y:

Figure 7: Tracing route from PC-X to PC-Y

8 Virtual Link Screenshot:

Figure 8: Virtual-link

9 Reference:
Praziale L, Britt T. D, Davis C., Forrester J. & Liu W. TCP/IP tutorial and technical overview [Online]
Verified at: http://www.redbooks.ibm.com/redbooks/pdfs/gg243376.pdf [Accessed:03/12/2013]
Techtarget.com (N/A) Interior Gateway Protocol [Online] Verified at:
http://searchsecurity.techtarget.com/definition/IGP[Accessed: 04/12/2013]
Cisco.com (N/A) OSPF virtual link [Online] verified at:
http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801ec9ee.s
html [Accessed: 30/11/2013]
Linfo.org (N/A) Distance vector routing definition [Online] verified at:
http://www.linfo.org/distance_vector.html [Accessed: 05/12/2013]
Computernetworking.com (N/A) Routing information protocol [Online] verified at:
http://computernetworkingnotes.com/routing-static-dynamics-rip-ospf-igrp-eigrp/riprouting.html[Accessed:06/12/2013]
Techtarget.com (N/A) ISIS (Intermediate System-Intermediate System Protocol) [Online] Verified at:
http://searchnetworking.techtarget.com/definition/IS-IS [Accessed: 08/12/2013]
Cisco.com (10/09/2012) Intermediate System-to-Intermediate System Protocol [Online] Available at:
http://www.cisco.com/en/US/products/ps6599/products_white_paper09186a00800a3e6f.shtml
[Accessed:30/11/2013]
Smith P. (09/04/2009) ISIS tutorials [Online] Verified at:
http://www.menog.org/presentations/menog-4/MENOG4-ISIS-Tutorial.pdf [Accessed:08/12/2013]

OSPF (Open Shortest Path First) Case Study: Anil Nembang

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