So welcome back, guys.
Now we will start learning the concepts of L3 routing.
So you can presume that this section of the course is the most important section of this course.
The concepts of L2 routing and L3 routing should be on your fingertips.
So we will go through the concepts of L3 routing in three phases.
In the first phase, we will discuss the topology setup that we will be using to understand L3 routing.
In the second phase we will understand that how network layer that is L3 layer works and what are its
operations.
And after building the necessary background behind Network Layer and its operations, we will go through
the step by step example of L3 routing.
So let us start with the discussion of the topology setup that we will going to use.
So you can see that in this topology we have four routers.
The name of the router is VM, VM one V m2 and M3.
Right.
You can see that all these four routers are connected via point to point links.
So this is point to point link, right?
The common subnet which is shared between the router and the router.
VM one is the subnet, one which has the network ID.
192.168.4.0 slash 24.
Similarly, subnet two is the common subnet which is shared by router VM one and router VM two.
And the network ID of this subnet is the following.
Similarly, we have a subnet three, which is a common subnet between router VM two and router VM three.
Now against each of these router in this topology, you can see this little box.
This little box is nothing, but it is ARP table.
So the number of entries in the ARP table of each router must be equal to the number of neighbors the
router has.
So it is for this reason that the router VM has only one entry in its ARP table router VM one has two
entries in the ARP table because router VM one has two neighbors, that is router VM and router VM two.
For the same reason, router VM two has two entries in the ARP table and the router VM three has only
one entry in its ARP table.
So just to recap, the ARP table contains nothing, but it contains the mapping between the Gateway
IP address and the Mac address.
You can see that each interface of each router in this topology is configured with a mac address.
So let us take an example for router.
For example, the router has an interface ethernet two which has a mac address ma.
Right.
And similarly you can see that all other routers in this topology has a mac address configured on each
of its physical interfaces.
Now let us discuss what are the subnets we have and what subnets are local and remote with respect to
each router in this topology.
So you can see that for router VM, the remote subnet is subnet two.
Right.
And the another remote subnet is subnet three.
So with respect to the router VM, the local subnet is subnet one.
Right.
So why these subnets are local and remote.
Just apply the definitions of remote and local subnets, which we have already studied, and you would
know that which subnet is local and which subnet is remote with respect to a given router.
Similarly with respect to the router VM one, subnet two and subnet one is a local subnet, whereas
subnet three is a remote subnet.
Right.
Same goes with router M2.
That is this router.
Subnet one is a remote subnet for router m2, whereas subnet two and subnet three both are local subnet
for router M2.
Right.
And for router m3 the subnet subnet two and subnet one both are remote subnet.
Where is subnet three is a local subnet for router M3.
Now next, let us discuss the routing table that we have configured in each of its router in each of
the routers routing table.
So let us discuss the routing table entries for the router VM.
So you can see that there are two entries in the routing table of router VM.
Now the first thing is that why do we have two entries?
We have two entries because in this topology there are two remote subnets with respect to the router
VM.
So remember in a routing table, we always install the entries for remote subnets.
So you can see that 192.168.5.0 slash 24 is a remote subnet, which is subnet three and 192.168.1.0.
Slash 24 is the remote subnet.
Subnet two, which is again a remote subnet with respect to the router VM.
So now we already know the semantics of L3 routing.
That is what is the meaning of this routing table.
Entries.
Now for router one, there is only one remote subnet, which is subnet three, and therefore we have
an entry for remote subnet three network ID.
Right.
Same is true for router vm2 and same is true for router VM three.
In this routing table entry you can ignore these fields.
It is of no use here and is not worth discussion.
So always remember that in the routing table of an L3 router, we always install the routing table entries
which dictates how to reach remote subnets.
Now you can see that for router VM one, we have an ARP table, for example.
So router VM one has two neighbors, that is router VM two and router VM.
So you can see that the router VM one has neighbor IP address, which is 192.168.4.2, which is an IP
address of the opposite end of this link.
And the Mac address of this physical interface is Ma.
So the router has IP address to Mac address mapping for next hop IP address.
Right.
Similarly for another neighbor, that is 192.168.1.2.
The router vm1 has an IP address to Mac address mapping.
Similarly, ARP tables for all other routers in this topology are populated.
So when a packet is routed in the network essentially for fields in the packet, we need to closely
understand what are those fields?
The source Mac address and destination Mac address present in the Ethernet header of the packet and
the source IP address and destination IP address present in the IP header of the packet.
So these four fields determines the path of a packet in the network.
And you can see that in this topology we have not deployed any L2 switch, right?
So Mac table is out of caution.
Also, we assume that ARP tables of all the routers in the topology are already populated, right?
So we have assumed that ARP tables are populated so that we can avoid the redundant discussion of ARP
resolution in this example, and we can focus only on L3 routing concepts.
So this is the topology I have explained.
We will run L3 routing example on this topology in order to explain the L3 routing concepts.