Now going forward.
Now, let us first discuss the steps which are involved in sending a frame from source host machine
A to L3 Gateway router.
So you can see in this topology, we have two VLANs involved that is Vlan 12 and Vlan 11 and therefore
we have created two interfaces on L3 router that is Svi, 11 and 12 and we have a mapping such that
Svi n binds with a vlan n that is Svi 11 has a binding with Vlan 11 and Svi 12 has a binding with Vlan
12.
You can see that this line is a trunk line and both the ends of this trunk line are operating in Vlan
11 as well as Vlan 12.
Right.
The other port that is port P five and port P six of L2 switch is operating in access mode.
So now you must understand this topology fairly easy.
So let us now go through the steps using which the host machine A shall be able to send a frame to the
Gateway L3 router.
So step number one, that is a finds that B's IP address 14 .1.1.4 belongs to a remote subnet.
Right?
Because a is network id is 13 .1.1.0 slash 24 and the destination IP address that is the IP address
of the host machine B is clearly do not belongs to this subnet.
Right?
So in step number two, A decides to send data using its default route.
Remember host machine make use of default route whenever it has to send data to a destination machine
present in a remote subnet.
Right.
So the default route, which is installed in the routing table of host machine A, it states that the
next hop or gateway IP address is 13 .1.1.1 and outgoing local interface is IFA.
That is this interface.
So now Host Machine A has decided that it has to send data to the Gateway L3 router out of local interface.
For right now in step number three for the host machine to send data to the Gateway router host machine
A needs Mac address of gateway IP address that is, it should know the Mac address corresponding to
IP address.
13 .1.1.1.
Right.
So to know the mac address the host machine a check its ARP cache to resolve ARP for default gateway
IP Address 13 .1.1.1.
Now here we have assumed that the ARP cache of host machine A is empty and therefore host machine a
launches ARP broadcast message out of local interface for to know the Mac address of 13 .1.1.1.
Right.
So here you can see that the host machine A has issued the ARP broadcast message to know the Mac address
of Gateway IP address.
13 .1.1.1.
Now we already know that how our broadcast message and ARP reply message travels in a network which
is Vlan enabled.
So you can see that when this broadcast request message enters the L2 switch, it will get tagged with
Vlan 12 and after that the L2 switch forwards the ARP broadcast message out of all the interfaces which
are operating in Vlan 12.
Now in this case you can see that only Port seven is operating in Vlan 12 and therefore the broadcast
message is forwarded out of Interface P7 of L2 switch.
So as a result the L3 router receives this ARP broadcast request message on interface P8 right.
Now here you can see that the Port P8 is configured with the Mac address, M-r and Port P8 is a real
physical interface of L3 router.
So therefore the L3 router returns.
The Mac address is equal to m r in arp reply to host machine a right so l3 router replies that the Mac
address for IP address 13 .1.1.1 is m r right.
So this ARP reply eventually received by the host machine.
A so when host machine A receives this ARP reply message, it means that host machine A has completed
its ARP resolution process and now host machine A knows the Mac address for IP address.
13 .1.1.1.
So now the host machine A has all the information it needs in order to send out the frame to Gateway
L3 router.
So as a result, in step seven, the host machine A prepares the frame in which the destination Mac
address is m r.
The source Mac address is the Mac address of local interface of host machine.
A The source IP address is the IP address of source machine A and of course the destination IP address
will be the IP address of destination machine B, so you can see that the host machine A has prepared
a frame, right?
So you can see in this diagram that this is the destination Mac address.
This is the source Mac address, this is the source IP address and this is the destination IP address.
I'm showing only the relevant fields of Ethernet header and IP header of this frame.
All other information which is present in rest of the headers of this packet is irrelevant.
So you can see that host machine A has transmitted out this frame out of local interface if.
So remember when the host machine a places this frame on the wire at that point of time that frame was
untagged.
When this frame enters the L2 switch on port b five, then this frame gets tagged with Vlan 12.
Right.
And after that the L2 switch makes use of its Mac address table in order to forward this frame out of
required ports.
So in this example, the L2 switch will forward the frame out of Port Number P7.
So as a result, this frame is received by L3 router only.
Right.
And now L3 router needs to forward the frame to host machine whose IP address is 14 .1.1.4 because it
is this IP address which is specified in the destination IP address field in the IP header of the packet
that is this field.
So, so far in our steps from 1 to 8, we we have discussed the mechanism that how the source host machine
A will be able to send a frame to the Gateway L3 router.
Now once the frame reaches Gateway L3 router, now it is the responsibility of L3 Gateway Router to
reroute the frame to its destination subnet.
So now rest of the steps is same as that of l3 router to Vlan routing because now l3 router has to forward
the frame such that the frame eventually reaches its destination vlan.
And we have already learned those steps in our previous module.
So we will go through those steps once again in order to complete this interval and routing steps.