OSPF- Open Shortest path first, had conveyed the details of OSPF routing explaination which comes under Dynamic routing protocols and also configured OSPF Multi-area with the help of CISCO Packet tracer. The persons who were Pursuing CCNA will gain more exposure on overviewing this.

1. OSPF
OSPF expands to Open Shortestpath first, comes
under IGP (Interior gateway Protocol) of Dynamic Routing category. This protocolroutes
InternetProtocol (IP) packets by gathering link-state information from neighboring routers
and constructing a map of the network. OSPF routers send many messagetypes including
hello messages, link state requests and updates and database descriptions.
Fig 1.1: Hierarchy chart of Routing Protocols
OSPF Overview
1. OSPF - Open ShortestPath Firstis one of the most important and mostused protocol
in the world of networking either at LAN side or WAN.
2. Enterpriselevel users usethis protocol for efficient communication of devices over
best possiblepaths.
3. ISP’s usethis protocolas IGP (Interior Gateway Protocol) for BGP to work fromone
end to another end.
OSPF Tables
1. Topology Table
2. Neighbor Table
3. Routing Table

2. Fig 1.2: A mutual connection between Four Routers
The above Schematic representation shows the connection of 4 Routers, and
each of the routers has a total of 1 Lakh routing units. If you take EIGRP and RIP protocol it
exchanges routing information for every 30 seconds. In OSPF caseif there is no network
exchange there will be no routing exchange, But OSPF has router refreshmenttime for
every 30 Minutes, that there will be an update between each router even if there was no
route.
1. Topology Table
Contains entire information of all routers in a single OSPF Network, with
the help of this we can know about all the database of the router.
2. Neighbor Table
Itonly Consists of Neighbor information of a particular router. For Eg:-
In Fig 1.1 Router 1(R1) only knows aboutthe info of Router 2(R2) and Router 4(R4).

3. 3. Routing Table
Consists of all the details of the interface connections, it can be
viewed as #do show ip-route.
OSPF Parameters
Tab 1.1: Details of OSPF Parameter
Why OSPF?
Under IGP, OSPF is preferred morethan EIGRP and RIP. Itis because at a
time EIGRP could connect 15 Routers, RIP could connect 250 Routers whereas OSPF could
connect N Number of Routers. Connecting these N number of routers in a single network
Description Default
Interface cost
Cost of each interface can be
specified. These values are
used to calculate the
shortestpath.
1
Hello interval
(seconds)
Often a router sends hello
messages to its neighbors. If
this parameter is too small,
more router traffic results.
This increases the risk of
dropped packets, which
could result in false alarms. If
interval is too big, topology
changes will take longer to
be detected, and router
dead interval may expire.
10
seconds
Router dead
interval (seconds)
Used to declare neighbor
routers dead when no Hello
messages havebeen
received. This interval should
be a multiple of the “Hello
interval”
40 seconds
Transmission
delay (seconds)
Estimated time to transmita
Link State Advertisement
(LSA) packet.
1.0 seconds
Retransmission interval
(seconds)
Time between LSA
retransmissions. Mustbe
greater than the expected
round-trip time between any
two routers in the network.
5.0 seconds

4. may causeconfuseand over burden to systems, to overcomethis issuewe separaterouters
into groups and that groups weredenoted as Areas. Among those areas, the areas with
most number of routing units will be denoted as Area 0 also the Backbone Area. All other
areas should connect with Area 0, the Non-Backbonearea connecting with Backbonearea
termed as ABR (Area Border Router).
AdministrativeDistance(AD) plays a pivotal role on selecting the path in
which the datagramneeds to travel, it was chosen on the base of lowestAD value. The AD
value of OSPF=110, EIGRP=90, RIP=120. Atbeginning if hostdon’t know its destination IP
but it needs to send the “Hello” packet, it will uses a Multicast Address=224.0.0.5.
OSPF Multi-Area Configuration
Fig 1.3: Configuration of OSPF Multi-area

5. Fig 1.4: Router 0(R0) Configured details.
Fig1.5: Router 1(R1) Configured Details

6. Fig 1.6: Router 2(R2) Configured Details

7. Fig 1.7: Router 3(R3) Configured Details
Fig 1.8: Router 4(R4) Configured Details
During the configuration in the ABR, it shows the
interface had attained full stage connection.
Verification: All the packets from PC0 needed to be received by PC1.

8. Fig 1.9: Ping Statusof OSPFMulti-Area Configuration.
*All the Configurations were done by using CISCO Packettracer