OSPF is a popular interior gateway protocol, frequently encountered by network engineers. Here's how to get it up and running in a network.

1. How to configure the basic OSPF?
OSPF is a popular interior gateway protocol, frequently encountered by network
engineers. Here's how to get it up and running in a network.
Open shortest path first, or OSPF, is a routing protocol that's based on open
standards, meaning any vendor can implement its features. For years Cisco, Juniper,
and other major networking vendors have coupled OSPF into their platform. When
you configure OSPF, there are a few fundamental requirements in order to exchange
routing information. To begin with, OSPF uses a Hello mechanism to detect
neighboring devices that run OSPF. Once detected OSPF attempts to form an
adjacency. Once an adjacency is established Link-State Advertisements (LSA) are used
to exchange routing information.
The Hello packet is one of the first things you should have a solid understanding of.
The following image shows the format of a Hello packet:

2. This packet is used by the Hello protocol, which is responsible for establishing and
maintaining neighbor relationships. The Hello protocol also ensures that
communication between neighbors is bidirectional. These packets get sent out of an
interface where OSPF is enabled on a periodic basis. If a router receives a packet and
sees its own address listed it knows that bidirectional communication exists. Once
bidirectional communication is established a Designated Router (DR) is selected if on
a multiaccess network. We're not going to cover that process in this article, however
we highlight it because it's an integral part of OSPF. After two devices neighbor up,
they synchronize Link-State databases.
To configure basic OSPF to form a neighbor adjacency on a Elins router perform the
following:
1.Identify the network interfaces that should run OSPF.
2.Identify the OSPF area
3.Issue the required commands to enable OSPF
Let's assume the following topology in which we are using OSPF area 5 and ABR5 and
R5 are pre-configured for us. We need to enable OSPF on R6.
Start by entering OSPF configuration mode. You'll need to identify OSPF by a process
ID. In this case, it's 1.
R6>en
R6#conf t
Enter configuration commands, one per line. End with CNTL/Z.

3. R6(config)#router ospf 1
R6(config-router)#
Next, issue the network statement for each interface that should run OSPF. According
to our network topology we will enable OSPF on interface g0/1 and g0/2. The
network statement format is as follows:
network X.X.X.X W.W.W.W area A
In this syntax, the X.X.X.X indicates the network address. The W.W.W.W is the
wildcard mask. The A is the area ID.
R6(config-router)#network 10.0.0.80 0.0.0.3 area 5
R6(config-router)#network 10.0.0.84 0.0.0.3 area 5
R6(config-router)#
A simple way to identify the wildcard mask is to subtract the subnet mask in the last
octet from 255. In this case 255 - 252 - 3 so our wildcard mask is going to end in .3.
We now need to verify that OSPF has neighbored up and synchronized databases. To
do so, issue the show ip ospf neighbors command.
R6#sh ip ospf neighbor
`
Neighbor ID Pri State Dead Time Address Interface`
192.168.0.10 1 FULL/DR 00:00:35 10.0.0.85 GigabitEthernet0/2`
192.168.0.9 1 FULL/BDR 00:00:31 10.0.0.81 GigabitEthernet0/1`
R6#`
As you can tell, we are receiving OSPF routes. In fact, we are receiving more routes to
destinations that we don't see in our topology. This is because we are only focusing
on a small part of the topology. What we don't see is that ABR5 is connected to
several other routers forming a backbone and exchanging routes between other
areas. This obviously leads to more questions about OSPF, however, as it stands we
have configured basic OSPF and can now route traffic on R6.
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