So guys, going forward, before diving deep into the concept of interval and routing, let us first
have a brief revision regarding traditional L3 routing that we have already learned in L3 routing module.
So you can see in this topology we have the host machine A which is present in network ID 13 .1.1.0
slash 24 and this host machine A wants to send data to the host machine B which is present in the network
with the network ID 14.1 .1.0 slash 24.
So basically the host machine A and host machine B are present in different subnets.
So this is the case of traditional layer three routing, where the two host machines present in different
networks wants to communicate with each other.
So whenever a host machine A needs to communicate with the host machine B, which is present in a remote
subnet, A sends a frame to its gateway router using default entry in its routing table.
So you can see that this is the host machine A and there is a default entry installed in the layer three
routing table for the host machine A and this default entry simply says that if you are sending a data
or a packet to a host machine which is present in a remote subnet, then your next hop IP address is
13 .1.1.1 and local outgoing interface is IFA.
So remember, we already know the semantics of a default route that any destination IP address matches
with this default entry.
So if machine A wants to send a data to any remote machine, that is to any machine which is present
in a remote subnet, the host machine A will going to use this default route entry in order to decide
where to forward the packet.
Now, as per this default entry, it has to forward the packet to the next hop, whose IP address is
13 .1.1.1.
Right.
So if it wants to send a frame which is destined to any IP address which is present in a remote subnet,
it has to send a frame to a L3 router.
Right?
So to do so.
Host machine a must know the Mac address for the default gateway IP.
Now the gateway IP is 13 .1.1.1.
Right.
So therefore the host machine A must have an ARP entry in its ARP cache and it must know the Mac address
corresponding to the gateway IP address.
Now you can see that 13 .1.1 is configured on an interface if R2 of an L3 router and the Mac address
of this interface is two.
Therefore, this ARP entry must be present in the ARP cache of host machine.
A.
If this entry is not present then ARP will launch a broadcast request to know to know the Mac address
for the Gateway IP address.
13 .1.1.1.
Right.
So once a has this ARP mapping A will be in a position to send a frame to l3 gateway router with destination
Mac address specified as Mr. two in the ethernet header of the frame.
Right.
So we are discussing nothing new.
We are just doing a revision briefly.
We have discussed all these steps in our traditional L3 routing.
So here you can see that the host machine a if do not know the Mac address of the next hop IP address,
then a launches the ARP broadcast request.
And for which IP address it should launch our broadcast request.
That information comes from the default routing entry.
So in interval and routing also we will going to have these two concepts.
So things are actually not very different in case of interval and routing.
In interval and routing, also the host machine will make use of its default route entry.
And we launch an hour broadcast request message to know the Mac address of the default gateway IP address.
So in case if you are finding this discussion a little unfamiliar, then it means that you need to go
back and revise the L3 routing concepts before proceeding forward.