So, guys, now we enter into the step number three.
That is flooding of the Neighborship database.
Right?
So this slide shows that following is the Neighborship database of the router, R2 and the router.
R1 and router R3 also have built their Neighborship database, though I have not shown that on this
slide.
Now the next thing that the router, R2 or any other router in the topology needs to do is to convey
every other L3 router about the contents of its neighborship database.
Right.
So for that, every router in the topology prepare a special packet called LSP.
LSP stands for Link State packet.
Right, and it copies the content of its entire Neighborship database into this LSP packet.
Right.
So remember, this packet is called link State packet.
So the router R2 creates its link state packet.
So every router then creates a special packet called link State packet in which it encodes all the information
of its local neighbor database.
Likewise, the router R1 as well as the router.
R3 will also create their respective link state packets from its local neighborship databases.
Right now, every router then floods its link state packets in the entire network.
The link state packets are representation of neighborship database plus optionally router can insert
more information into its link state packets.
So now we will see that how the flooding of the link state packet happen.
So suppose the router R1 has its link state packet.
Now after the router has prepared its link state packet, its next job is to ensure that every other
router in the topology receives this link state packet.
Similarly, same responsibility is there on the router, R2 and router R3.
They also want their respective link state packets to be communicated to every other router present
in the topology.
So router r1 will send its link state packet to all its directly connected neighbors.
In this case the router R2 is the only neighbor of the router R1.
Therefore router r1 will send its link state packet to the router r2.
Right.
When the router R2 receives the link state packet of the router r1, it does two things.
The first thing is that that it will going to store the link state packet of the router r1 in its local
memory.
Right.
So it has stored the link state packet of the router r1 in its local memory.
And the second thing the router R2 will do that it will going to forward the link state packet to all
its directly connected neighbors except the neighbor from which the link state packet is received.
Right.
So you can see that the router r2 propagates further.
The link state packet of the router r1 to its directly connected neighbor and the router R3 does the
same thing.
It receives the link state packet of the router R1 and store it into its local memory.
Right.
And if the router R3 has further directly connected neighbors, then the router R3 would flood this
new link state packets to its directly connected neighbors, except the neighbor from which the link
state packet was received.
So doing this exercise, the end result is that that every routing device receives the link state packets
of every other device in the network.
So if every router receives the link state packet of every other router in the topology, the link state
packet database of router, R1, R2 and R3 would be same.
Right.
And they will contain the link state packet of the router, R1, R2 and R3.
Right.
Now the collection of all link state packets of all routing devices is called link state packet database
and it is nothing, but it is just a network graph.
So it simply means that every routing device in the topology now has built an input graph which represents
an actual physical topology.
Right.
How this link state packet database represents a physical topology.
Let us try to understand that.
So suppose this is the physical network topology that we started our discussion with.
So suppose the router has generates its own link state packet and install its link state packet into
its local link state packet database.
Right.
So router s knows that A is its neighbor as well as the router D is its neighbor.
Right.
So remember this diagram is a graph data structure which exists in the local memory of the routing device
as.
And here we are trying to see that how this data structure builds up to represents the entire network
topology as the routing device as receives packet from more and more devices present in the physical
topology.
Now suppose the router asks receives the packet of the routing device A so in the packet of the routing
device a A would convey that B is its neighbor as well as the router.
D is its neighbor.
Right.
And since the router S is also the neighbor of the routing device, a therefore A would convey in its
packet that the router s is also its neighbor.
Now going further, suppose the router s receives the packet of the routing device D Right.
So D will convey that A is its neighbor and C is its neighbor.
So you can see that the graph data structure in the local memory of the router as is growing.
And now when the router s receives the packet from the router, B then B would convey that C is its
neighbor as well as the router.
A is its neighbor.
Right.
And finally, when the router s receives the packet from the router, C, then C would convey that B
is its neighbor and the router D is its neighbor.
Right?
So you can see that this particular graph data structure, which is built into the local memory of the
routing device as exactly resembles with the physical topology.
Right?
So similarly, every other routing device in the physical topology receives the LSB packet of every
other device of the physical topology.
And the end result is that that every device of the physical topology builds up the input graph data
structure in its local memory.
So every node builds up the same link state packet database.
It means that every node has the same view of the physical network topology in the form of internal
graph data structure.
Right.
Now every node can apply certain algorithms on this input graph data structures in order to compute
networking bias results such as routing table.
So finally, we have achieved our goal of phase one that every device of the physical topology now has
an internal data structure which represents the physical topology itself.
So concluding the phase number one.
You can see in this slide that the routing device R1 has its link state database, which is nothing
but an lsps of router, R1, R2 and R3.
Right.
Similarly, Router R2 has a link state database and the router R3 also has a link state database.
Right.
The content of link state packet database is exactly same on every routing device in the topology.
Every routing device has the same view of network topology because their link state packet database
is same.
Every routing device then runs several algorithms on its local network graph that is link state packet
database to compute results such as its local routing table.
And this completes our phase one.