So guys before actually learning that how L3 router delivers the traffic to the host machine present
in a particular Vlan, we need to actually build up some prerequisite knowledge.
So let us first study that what exactly router possession is.
So you can see in this diagram we have an L3 router which has three physical interfaces.
And on these three physical interfaces we have configured an IP addresses.
Now if you apply the mask value on these IP addresses, you will get the network ID of these physical
interfaces.
So we say that this L3 router actually hosts three local subnets, the subnet one, subnet two and subnet
three.
And you can derive or calculate the network ID of each of these subnets.
So you can see here is the network ID of each of these three subnets.
So in other words, we say that this L3 router is in a position of these three subnets, right?
So note the terminology.
We say that this L3 router is in a position of these three subnets.
Now it means that this router should receive the traffic for destination IP address is equal to member
of any subnet possessed by this router.
Now what does that line mean?
So let us take an example to understand this line.
For example, this router, you can see that this router hosts the subnet one, right?
And the network ID of this subnet one is ten .1.1.0 slash 24.
So on this subnet you will have many host machines and the IP address of each of these host machines
would be of the format.
Ten .1.1. x slash 24 where x is any value.
From 1 to 255.
And of course, X cannot be equal to four because four is already used.
So this line simply means that this router should receive the traffic which is destined for IP address.
Ten .1.1. x because the host machine will be present in the local subnet which is directly attached
to this L3 router.
So once this router receives the traffic for the host machine which is present in one of its local subnet,
this L3 router will take care to deliver the traffic to the correct host machine using L2 routing.
Right.
So this router must receive the traffic for destination IP address is equal to member of any subnet
possessed by this router.
Right.
So this router can receive a traffic for IP address.
11 .1.1. x and for IP address 12 .1.1. x.
Right.
Because this router hosts the subnet in which these IP address is a member of.
Right.
So now going further.
Now let us assume that this router has only three physical hardware interfaces.
And remember, IP address and mask value are always configured on interfaces only.
Now this router has three physical interfaces.
It means that you cannot make this router in possession of more number of subnets.
That is more than three subnets, right?
So this is the limitation.
You are limited by the number of physical interfaces this router has.
So what will you do in order to make this router possess more number of subnets?
Now of course you will say that you will attach more physical interfaces to this router and assign an
IP address to those interfaces so that router becomes in possession of more number of subnets.
But okay, I impose a constraint that interfaces are actually expensive live and you really cannot buy
and attach more number of physical interfaces to this L3 router.
So what will you do?
Your goal is to make this router in possession of more number of subnets, but at the same time you
cannot attach more number of physical interfaces to this router.
So the solution is if you cannot attach more number of physical interfaces to the L3 router, then instead
of physical interfaces, use logical interfaces and assign IP address and mask value to them like you
do to physical interfaces.
So these logical interfaces are given special name and they are called switch virtual interfaces.
Now note the word virtual virtual means something that do not exist, but we are simulating it as if
it exists, right?
So these logical interfaces are called switch virtual interfaces or Svi.
And remember Svis are not physical or hardware interfaces, they are just software based interfaces
like loopback interfaces.
I have already discussed in detail about loopback interfaces, so Svi has the same property as loopback
interfaces.
That is, Svi.
Interfaces are logical interfaces.
They are configured with IP address and mask value and they are not used for any kind of data exchange.
So if you so I would suggest if you are not able to recall, go to the previous module and learn about
and revise about loopback interfaces because the property of loopback interfaces are same as that switch
virtual interfaces.
So remember, as we are interfaces are logical interfaces and because they are logical interfaces,
you can create as many interfaces on the L3 router as you want.
You don't have to go to the market and buy X, Y, and of the day there is a piece of software that
runs on the L3 router and that software creates the illusion of physical interface of L3 router.
But actually that is logical interface.
So you can create as many svis you want on an L3 router.
Now going forward.
So let us create SB ten, SB 20 and SB 30 on router and assign IP address and mask value to them.
So you can see in this diagram we have created three SB interfaces on the L3 router and we have assigned
an IP address and mask value to these interfaces.
Just like you assign IP address to any physical interface of L3 router.
Likewise, you assign the an IP address to the SB interfaces of an L3 router.
We have also taken one loopback interface also.
That is, we have created one loopback interface on this router and we have assigned an IP address and
mask value to this loopback interface.
Now we already know that.
Why do we assign the mask value of 32 to the loopback interface?
Right now you have created three interfaces and one loopback interface and your L3 router already have
three physical interfaces.
So in total your router is now in possession of seven subnets.
You can also derive the network ID or subnet ID from the IP addresses configured on interfaces.
So you can see that as an interface.
Derive the network ID or subnet ID 50 .1.1.0 slash 24.
Similarly, if we have network ID for Svi 20 and we have network ID for Svi 30, so you can see that
using the concept of svis you can make an L3 router is in possession of as many subnets as you want
without attaching any real physical interface or hardware to an L3 router.
Now since that your L3 router is in possession of these seven subnets, so this router must receive
the traffic whose destination IP address is equal to member of any subnet possessed by this L3 router.
Right.
So it means that in addition to real physical subnet, this router would receive the traffic which is
destined for IP address 50 .1.1. x or 60 .1.1. x, where x value could be anything from 1 to 255.
Right.
And of course the router can also receive the traffic that is destined to itself.
Right.
So using interfaces, you have actually created more number of subnets on this L3 router, and this
L3 router should receive the traffic which is destined for a host machine present in those subnets.
Right.
So going forward.
So if we have an L3 router and on this L3 router, you have created three interfaces.
Now when this router receives the traffic with destination IP address is equal to 11 .1.1.3.
So it means that this traffic is assigned to a host machine which is present in this subnet.
That is 11 .1.1.0 slash 24.
So this router should forward such a traffic on its outgoing interface.
IFP Right, and it should use L2 routing to deliver the traffic to this particular host machine whose
IP address is.
11 .1.1.3.
Right.
In the second case, when this router receives the traffic for destination IP address 100 .1.1. 1 or
11 .1.1. 4 or 12 .1.1. 4 or 10 .1.1.4.
So you can see that all these IP address are actually the IP address which are actually configured on
the physical interfaces of this router, right?
So it's an exact match.
That is 100 .1.1.1 matches with the loopback IP address and 11 .1.1.4 matches with the IP address of
interface, IFB and so on.
So whenever the destination IP address of the packet matches with the IP address of one of the local
interface of L3 router, including the loopback interfaces, then that particular traffic is destined
to the router itself.
That is, router itself is a destination for such a traffic and therefore router itself will consume
the traffic because there is an exact match.
So all these we have already learned in the concept of L2 and L3 routing, but what will the L3 router
do if it receives the traffic for example with destination IP address?
Say 50 .1.1. x.
Now you can see in this diagram that a router has a network ID, which is 50 .1.1.0 slash 24 and this
destination IP address is a member of that particular subnet.
But in this diagram, where is that subnet?
Right.
So here the question is that what will router do if it receives the traffic?
Say for example, it receives a traffic for 50 .1.1. ten.
Now of course this IP address is a member of this subnet that is 50 .1.1.0 slash 24.
But to which host machine this L3 router will forward this traffic to.
Right.
So router has to forward the traffic to machine present in directly connected local subnets.
Now in this case the directly connected local subnet is 50 .1.1.0 slash 24, but we don't see any physical
subnet corresponding to network ID.
50 .1.1.0 slash 24.
So where will this router will forward the traffic to and where are those subnets?
Right, so we have so many questions.
So let us try to understand and answer these questions.