So, guys, now going forward now you already know that we can make L3 router in possession of more
number of subnets by creating SPI interfaces.
Right.
So let us assume that we have already created three interfaces on this L3 router and these three interfaces
are Svi, ten, Svi 20 and Svi 30 and you can see that we have assigned IP addresses to these Svi interfaces.
Now in order to prepare the L3 router to possess the capabilities to forward the traffic to a particular
host present in a Vlan, we need to do certain configuration on L3 router.
So we will going to discuss what are those configuration.
So first of all, you have to take one physical interface of a router.
In this example, suppose we have taken a physical interface of.
And configure this interface as trunk.
So we have taken one physical interface of L3.
Router and we have configured it as trunk.
Now configure this physical interface to operate in Vlan ten, Vlan 20 and Vlan 30.
So we are assuming that in our topology there will be three VLANs and therefore we will configure this
physical interface to operate as trunk and configured under three VLANs that is Vlan ten, Vlan 20 and
Vlan 30.
Now in the next step, bind the interfaces to corresponding VLANs and do this 1 to 1 mapping.
So you have created three interfaces that is SB ten, 20 and 30 and you have created ten VLANs on an
L3 router which is Vlan ten, Vlan 20 and Vlan 30.
Now you need to do the binding.
Binding means 1 to 1 mapping.
So you can see that Vlan ten is bound to SB ten will n 20 is bound to SB 20 and Vlan 30 is bound to
SB 30.
Right?
So every Vlan that you have created on L3 router must have a binding with exactly one SB interface that
you have created on l3 router.
This is called Vlan, SB binding.
Now we will try to understand that why we have done this binding.
So by binding SB and Vlan together we LAN actually borrows the network ID of spies, right?
So for example, Vlan ten network ID will be 50 .1.1.0 slash 24.
Why?
Because you have bound will n ten with SB ten and the network ID that can be derived from the IP address
configured on SB ten is 50 .1.1.0 slash 24.
Right.
So through binding VLANs has actually borrowed the network ID of Svi interfaces.
Right?
So Vlan ten network id will be 50 .1.1.0 slash 24.
And therefore all host machines present in Vlan ten must be configured with IP address.
50 .1.1. x slash 24 so that all those host machines derive the correct network ID.
So we are doing nothing new.
We are just doing basics.
That is, we already know that every subnet must have a network ID, right?
And VLANs are nothing, but they are also subnets and therefore VLANs should also have some network
ID, right.
So by doing so by doing the binding of VLANs with Svi interfaces, we are actually configuring the network
ID for VLANs.
Similarly, network ID for Vlan 20 and Vlan 30 will be 60 .1.1.0 slash 24 and 70 .1.1.0 slash 24.
Right, so VLANs derive or borrow their network ID from the interfaces with which they have been bound.
So guys, after doing the Vlan and interface binding or L3 router is now in a position to actually forward
the traffic to a host machine present in one particular Vlan.
So essentially router has to perform four basic steps in order to determine that how it should forward
the traffic so that traffic eventually reaches the host machine present in one of the villains.
So let us discuss the steps.
So if our L3 router receives a traffic, for example for destination IP address 60 .1.1. ten.
Right.
So let us assume that this is our L3 router.
And on one of the interface this could be any interface.
So on one of the interface from internet, let us suppose this L3 router receives a traffic whose destination
IP address is 60 .1.1. ten.
Right.
So when this l3 router receives a traffic or a packet with destination IP address 60 .1.1. ten.
So the L3 router checks that in, which is why the destination IP address is a member of.
So in this case it will be Svi 20 because the network ID that is derived from the IP address that is
configured on Svi 20 is 60 .1.1.0 slash 24 and the destination IP address in the packet lies in this
subnet.
Right?
So router selects Svi 20.
Now in the second step router checks that with which Vlan the Svi 20 logical interface has a binding.
So in this case it is Vlan 20.
Right.
Now as a next step, the router takes the packet with vlan id 20 that is router inserts 802.1 q vlan
header in the ethernet header of the packet that it has received.
Right.
And in the final step router forwards the packet out of all physical interfaces which are operating
in Vlan 20.
Router tags the packet with Vlan ID 20 and forwards the packet out of all physical interface which are
operating in Vlan 20.
Now in this diagram, the only physical interface of an L3 router which is operating in Vlan 20 is Ieft.
Right.
So the router forwards the packet out of interface Ieft only.
So remember when the packet leaves the L3 router on a physical interface Ieft this packet will be tagged
with vlan id 20.
Right.
So these are the basic four steps which are performed by an L3 router to deliver the packet to a host
machine which is present in a Vlan.
Right.
And remember that any packet that the L3 router receives from the outside world that is internet, that
packet will always be untagged.
It will never have any 802.1 header already.
The packet becomes tagged only when l3 router forwards the packet on a Vlan.
Right.
So now we know all these four steps which needs to be performed by an L3 router in order to route the
traffic in a Vlan.
So now let us demonstrate these four steps with the help of an example.