This document provides an overview of a seminar presentation on Open Shortest Path First (OSPF) routing protocol. The presentation covers the basic concepts of OSPF including its use of the Shortest Path First algorithm, areas, router types, header format, and hello packets. It also gives examples of OSPF configuration and important terms like loopback interfaces, designated routers, and authentication. The summary highlights both the processor intensive nature of OSPF but also its advantages like hierarchy, link state design, and support for VLSM.

1. Seminar Presenting On
Open Shortest Path First (OSPF)
Abdullah Mukhtar
13SCSE201018
M-TECH (CSE) BATCH -1

2. Overview
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Introduction
Simple basic configuration
OSPF configuration
Header format
Important term used in OSPF
Hello packet
Drawback & Benefit

3. Introduction of OSPF
1. The OSPF protocol is a link state Protocol that handles routing for IP traffic.
2.
Open Shortest Path First (OSPF) uses the Shortest Path First (SPF) algorithm, to provide
a loop-free topology.
3. Open Shortest Path First (OSPF) provides fast convergence with triggered, incremental
updates via Link State Advertisements (LSAs).
4. Open Shortest Path First (OSPF) is a classless protocol and allows for a hierarchical
design with VLSM and route summarization.
5. The two important concepts in case of OSPF are Autonomous Systems and Areas.

4. Important Term Related with OSPF
Loopback Interface:- A loopback interface is a logical, virtual interface on a router
o Router(Config)#int loopback 2
o Router(Config-if)#ip address 200.0.0.10 255.255.255.0
Area Border Router (ABR):-An Area border router (ABR) is a router that connects one or more
OSPF areas to the main backbone network. It is considered a member of all areas it is connected to.
Internal Router:-An Internal Router is a router that has only OSPF neighbour relationships with routers
in the same area.
Backbone Router:-Backbone Routers are part of the OSPF backbone. This includes all area border
routers and also routers connecting different areas.
Designated Router (DR) and Backup Designated Router (BDR):-A DR is the router interface elected
among all routers on a network segment, and BDR is a backup for the DR. DRs are used for reducing
network traffic by providing a source for routing updates. The DR maintains a complete topology table of
the network and sends the updates to the other routers via multicast. All routers in an area will form a
slave/master relationship with the DR .

5. OSPF Areas
Group of contiguous nodes/networks
Per area topology DB
Invisible outside the area
Reduces routing traffic
Backbone Area is contiguous
All others areas must connect to the
backbone
Virtual Links
Area 2
Area 3
Area 0
Backbone Area
Area 4
Area 1

6. OSPF protocol configuration
Router01(config)#interface fa0/0
Router01(config-if)#ip address
172.16.0.1 255.255.0.0
Router01(config-if)#no shutdown
Router01(config)#interface s0/0
Router01(config-if)#clock rate 64000
Router01(config-if)#ip address
172.17.0.1 255.255.0.0
Router01(config-if)#no shutdown
Router02(config)#interface s0/0
Router02(config-if)#ip address
172.17.0.2 255.255.0.0
Router02(config-if)#no shutdown
Router03(config)#interface s0/1
Router03(config-if)#ip address
172.19.0.2 255.255.0.0
Router03(config-if)#no
shutdown
Continue…

7. OSPF protocol configuration
Required Commands:
Rtr(config)# router ospf process-id
Rtr(config-router)#network address wildcard-mask area area-id
Optional Commands:
Rtr(config-router)# default-information originate
(Send default)
Rtr(config-router)# area area authentication
(Plain authen.)
Rtr(config-router)# area area authentication message-digest
(md5 authen.)
Rtr(config)# interface loopback number
(Configure lo as RtrID)
Rtr(config)# interface type slot/port
(int Fa0/0)
Rtr(config-if)# ip ospf priority <0-255>
(DR/BDR election)
Rtr(config-if)# bandwidth kbps
(Modify default bandwdth)
RTB(config-if)# ip ospf cost cost
(Modify inter. cost)
Rtr(config-if)# ip ospf hello-interval seconds
(Modify Hello)
Rtr(config-if)# ip ospf dead-interval seconds
(Modify Dead)
Rtr(config-if)# ip ospf authentication-key passwd
(Plain/md5authen)
Rtr(config-if)# ip ospf message-digest-key key-id md5 password

8. Header format of OSPF Packet
0
8
Version
Type
16
31
Packet length
Router Id
Area Id
Check sum
Authentication type
Authentication
OSPF
common
header
Authentication
Data
Fig.
OSPF packet
body
OSPF common header precedes each OSPF Packet

9. Configuring a Loopback Address
Rtr(config)# interface loopback 0
Rtr(config-if)# ip add 10.1.1.1 255.255.255.0
 Automatically are “up” and “up”
 Very useful in setting Router IDs as they never go down.
 RouterID is used to identify the routers in the OSPF network
– IP address configured with the Router-ID command
– Highest loopback address
– Highest active IP address
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Important for DR/BDR elections unless you use the ip ospf priority command
Extra: Also, useful to configure “virtual” networks that you can ping and route as if they
were attached networks.

10. The Hello Protocol

When a router starts an OSPF routing process on an
interface, it sends a hello packet and continues to send
hellos at regular intervals
◦ The rules that govern the exchange of OSPF hello packets are
called the Hello protocol
◦ Type field is set to 1 to indicate packet contains hello
information

11. More on Hello Packets…
• Hello packet timing…
– sent every 10 seconds by default on broadcast multi-access and point-to-point
networks
– 30 seconds by default on interfaces that connect to NBMA networks, such as
Frame Relay

12. Disadvantage and Benefit
Disadvantages:OSPF is very processor intensive.
OSPF maintains multiple copies of routing information, increasing the amount of memory needed.
OSPF is not as easy to learn as some other protocols.
Advantages:OSPF is hierarchical, using area 0 as the top as the hierarchy.
OSPF is a Link State Algorithm.
OSPF supports Variable Length Subnet Masks (VLSM).
OSPF uses multicasting within areas.

13. THANK
YOU