So guys, now we will going to discuss step number one and step number two in a single slide.
That is how router exchange hello packets with each other and build up the neighborship database.
So to illustrate it with the help of an example, you can see a physical network topology on the screen,
which consists of three devices R1, R2 and R3.
Right?
All these three devices are layer three routers.
Each of these devices is configured with their respective loopback addresses.
Right?
And the interface of each of these devices is configured with a name and an IP address.
Right.
Now as a step number one, each of these devices exchange hello packets with each other.
Right.
Each of these layer three devices, that is R1, R2 and R3 exchange hello packets with their respective
neighbors.
For example, the router R1 will send a periodic hello packet to the router, R2 and router.
R2 also sends periodic hello packet to the router R1.
Note that these hello packets are exchanged only between directly connected routers.
Right.
It will not happen that the hello packet of the router r1 would read R3.
Hello packets are non-routable packets, meaning that they stay in the same subnet.
Similarly, routers, R2 and R3 will going to exchange their respective hello packets with each other.
So in networking exchange of the hello packets is very, very common activity.
Many networking protocols, be it STP that is spanning tree protocol or be it OSPF, that is shortest
path first protocol or be it BGP or be it ISIS or ET cetera does the same thing.
They exchange hello packets in order to know their neighbors.
Now let me show you what should be the content of a typical hello packet.
So you can see that this is the content of a hello packet which is generated by the router R1 on the
interface Ethernet zero.
That is.
This is the content of this.
Hello packet.
Right.
The minimal information which the router needs to specify in this.
Hello packet is the name of the router itself.
That is R1, the IP address out of which this hello packet is being sent out.
That is ten .1.1.1 slash 24.
The name of the interface out of which this hello packet is being sent out and router can also specify
its own loopback address in the hello packet and et cetera.
ET cetera means that router can specify much more information in its Hello packet.
So what information needs to mention in the hello packet?
Depends on the functionality that the routing device is exchanging hello packets wants to achieve.
Similarly, similar will be the content in the hello packet that is generated by the router R2 on the
interface ethernet one and the same goes for the router R2 and the router R3.
Right.
So if you notice the contents of the hello packet is nothing but the information in the hello packet
tells something about the router which is generating the hello packet by hello.
It simply means that the router is trying to greet its neighbor.
Right?
That hey, I am router r1.
My loopback address is 122 .1.1.2 and I'm connected to you via interface ethernet zero, which has an
IP address.
Ten .1.1.1 slash 24.
Right.
So basically using hello packet, the router R1 is telling the router R2 that how am I connected to
you that I'm your neighbor and following are my credentials to you.
That is, I'm your neighbor and following is my information which is relevant to our neighborship relationship.
So in this topology router R1 has one directly connected neighbor that is R2, whereas the router R2
has two directly connected neighbor that is router R1 and R3.
Right.
And router R2 is receiving hello packets on its interface Ethernet one and on its interface Ethernet
two.
Right.
So it means that the Neighborship database of the router R2 would have three would have two entries,
right?
Each of these two entries correspond to a neighbor to a router, R2.
Right.
So this table shows the neighborship database of a router R2, which the router builds up in its local
memory as a result of receiving hello packets from its directly connected neighbors.
So if you look up at the row number one, it simply says that R1 router is the neighbor of the router
R2, which is connected with interface Ethernet zero and the local interface of the router R2 to which
it is connected is Ethernet one and the IP address that is configured on the neighbor side is ten .1.1.1
slash 24 and the IP address which is local, is ten .1.1.2 slash 24 And following is the loopback address
of the neighbor, which is the loopback address of the router.
R1.
Right.
Similarly, the second entry in this table is an entry of the neighborship of the router R3 with respect
to the router R2.
Right.
Remember, this table represents the neighborship database of the router R2, and similarly, every
L3 device in the topology builds its own neighborship database as a result of exchanging hello packets
with its directly connected neighbors.
So this completes the step number one and two.
And it's pretty much very simple, right?
Now we will see that once the router builds the Neighborship database, what does it do with this database?