Now continuing from the previous slide host machine D has successfully received the broadcast message
and now host Machine D prepares the ARP reply.
So the content of the ARP reply will be that source Mac address will be D right destination Mac address
will be the Mac address of ARP broadcast message originator, so it will be the Mac address of the host
machine C and the destination IP address will be the IP address of our broadcast request originator,
which is 12 .1.1.6.
So this will be the content of ARP reply.
So l2 switch to receives the ARP reply message on interface B3 and because it is receiving the ARP reply
message on interface B3, which is configured to operate in Vlan ten and is configured to operate in
access mode.
The ARP reply message gets tagged with Vlan ID ten now l2 switch to does Mac learning.
It inserts the following entry in its mac table that is Vlan ID ten and the source Mac address that
is specified in our reply is MD and outgoing interface will be port B3.
Alto switch to check its mac table entry for Vlan.
ID is equal to ten and mac address is equal to MC as a key.
Why it is looking the table entry for this because the Vlan because the ARP reply message is tagged
with Vlan ID ten and the destination Mac address in the ARP reply message is Mac.
So you can see that L2 switch two will find a matching entry and it knows that it should forward the
packet out of interface P1.
So the ARP reply message will be forwarded on the trunk line out of the interface P1 by l2 switch two.
Right, so l2 switch two forwards the frame out of interface P1 only.
Why?
Because of this matching entry.
L2 switch one receives ARP reply message on interface.
P6 l2 switch one does Mac learning and it inserts the entry ten AMD and P6 in its Mac address table
right now.
L2 switch one Check its Mac table for Vlan id is equal to ten and Mac address is equal to MK as a key,
so.
L2 switch one will find this matching entry and now.
L2 switch one knows that it has to forward ARP reply message out of interface P8.
Now l2 switch one forwards the reply message out of interface b8 only and also l2 switch one and tags
the ARP reply frame.
Eventually she receives the reply and it updates its ARP cache.
So in the ARP cache host machine, C inserts the following entry.
That is 12 .1.1.2 and MD.
It means that Host Machine C knows that the Mac address for IP address 12 .1.1.2 is empty.
Now after step number 23, the ARP resolution has been completed.
Right.
And now it's a time for actual data transfer.
So host machine C knows that these Mac address now so now host machine C is in a position to prepare
the frame.
So in that particular frame, the destination Mac address will be the source.
Mac address will be the source IP address will be the IP address of host machine C and destination IP
address will be the IP address of host machine D.
So you can see that a complete packet can be generated using this information.
Source Mac and Destination Mac will go in Ethernet header and source and destination IP address field
will go in IP header.
So in continuation from the last slide l2 switch, one receives the packet that is prepared by the host
machine C and again it tag it with Vlan ten.
Now this time this packet or this frame is actually a data packet.
So L2 which one does Mac learning now in step number 26.
L2 Switch one sees that the packet has been tagged with Vlan ID ten and the source mac address in the
packet is Mac.
So it checks up its mac table and look up using the key vlan id is equal to ten and mac address mac.
Now such an entry is already present.
Therefore L2 switch one does nothing, but it only refreshes its entry in the Mac table.
Now in step number 27 l2 switch one Check its Mac table entry for Vlan id is equal to ten and the destination
Mac address in the packet is empty.
So here is a little correction.
It is MD so l2 switch one finds the matching entry and l2 switch one now knows that it has to forward
this packet out of interface P6.
Right.
So the data packet has been forwarded by L2.
Switch one out of interface P6 and remember that this packet or this frame is tagged with Vlan ID ten.
Now in step number 29 L2 is to receives the frame on interface.
P1 right l2 switch to again does the Mac learning.
Now for this mac learning the villain ID with which the packet has been tagged is ten and the source
mac address of the packet is mac.
Right.
So l2 switch to will look up in its mac table two and find the matching entry.
Right.
Since this entry is already present.
Therefore l2 switch to only refreshes this entry.
Right.
Now in step number 31 l2 switch to check its mac table for Vlan id is equal to ten and destination mac
address is specified in the frame is MD.
So this is the matching entry which l2 switch to will find in its mac table.
So l2 switch to now knows that it has to forward the packet out of interface P3.
Right.
So l2 switch to forwards the packet out of interface three and the frame gets untagged as per the rules.
So you can see that the machine eventually receives the data packet, which is an untagged data packet.
And in step number 34, the transmission of the actual data from source host machine C to destination
host machine D is completed.
So the thing to note here is that that I have also elaborated that how ARP resolution will take place
in a Vlan based network and once the ARP resolution has been completed after that, only the actual
data transfer takes place.
So step number one to step number 23 are actually ARP resolution steps.
And then from step number 24 to step number 34 is actually the steps which involves actual data transfer
from host machine C to host machine D.
Also one thing to note here is that that whenever the L2 switches receives any packet from any interface.
L2 switch does two things.
The first thing that the L2 switch does is to do Mac learning.
So Mac learning is done by inspecting the 802.1 q vlan header in the packet as well as the source Mac
address field in the packet.
Right.
After that, the second thing that the L2 switches do is to.
Take a decision where to forward the packet and in order to take a decision.
The L2 switch has to inspect the 802.1 Vlan header in the packet as well as the destination mac address
field in the packet.