In this Nugget, we're going to take a look
at additional scenarios regarding
certain types of traffic going to a switch network
to predict what the MAC address tables look
like on the switch after that specific traffic is
sent through the network.
And for this scenario, let's predict
what the MAC address tables would
look like on the switches.
When we do a ping from PC-1 1 to PC-4
we're starting off with clean MAC address
tables and clear ARP caches on all of our devices.
And as the switch is building these tables,
we know it's building it based on these source MAC addresses
in frames as they come into the switch.
So in this discussion we can just
focus on traffic going into the switch.
So, a ping from PC-1 to PC-4.
So PC-1 is here.
PC-4 is here.
They're in the same VLAN.
And we have a trunk going between the two switches.
If PC-1 doesn't know the layer 2 address of PC-4,
it's going to do an ARP request.
And that traffic would go in on this port
right here on the switch.
And that would be one of our entries in the MAC address
table.
It would be the MAC address of PC-1, ending in 6801,
that's reachable off Gig 01 on VLAN 100.
So it's going to be VLAN 100, MAC address of PC-1 ending
in 6810, and that's on Gig 0/1.
That traffic would also be forwarded over to Switch-2.
It would be received on Gig 0/1.
And so on Gig 0/1 on Switch-2, it would also know about
the MAC address of PC-1 ending in 6810,
which is in VLAN 100 because of the 802.1Q tag that Switch-2
saw on that frame as it came in.
And then when PC-4 responds with an ARP reply,
that's going to go into the switch here on Gig 1/1.
So that's Gig 1/1 right there.
That's the MAC address of PC-4 at 6804.
And that access port, Gig 1/1, is in VLAN 100.
Then because Switch-2 is forwarding a frame destined
for 6801, based on its MAC address table,
it knows it needs to forward it that way.
So it sends the frame this way that's received by Gig 0/0
on Switch-1.
So on Gig 0/0, Switch-1 now learns
about the MAC address of PC-4 from its source MAC address.
And this MAC address is ending in 6804.
And that's also in VLAN 100, because the frame going this
way also was tagged with the 802.1Q tag,
indicating that frame belonged to VLAN 100.
And then that response would make it up to PC-1.
And then as PC-1 sends the ICMP ping request over to PC-4,
there's nothing new to be learned
by either switch there from PC-1 source MAC address,
nor for the ping replies.
It goes back, because Switch-1 and -2 have already
memorized PC-1 and PC-4's MAC addresses, respectively.
I've got some aliases in place.
One of them is called clean.
And if we issue that command, it'll
clear out the dynamically learned addresses
in the MAC address table.
I've also got an alias called smt, which will
show the MAC address table.
So on both switches, we'll issue the alias
clean to clear the MAC address tables,
and then we'll go to PC-1 and do our ping.
So we're here on PC-1.
We'll do a show ARP, just to validate
there's nothing in the ARP cache on this local computer.
And we'll do our ping.
And PC-4 is at 10.100.0.4.
We'll issue that ping.
And that looks great.
Now we'll go take a look at the MAC address tables
over on Switch-1 and Switch-2.
So over on Switch-1 we'll issue the command show
mac address dash table, using the alias of smt
on this Cisco switch.
And we'll do the same over here on Switch-2.
And that indeed does match our prediction here on Switch-1.
So we have the MAC addresses of PC-1 and PC-4,
respectively, both in VLAN 100, reachable via ports Gig 0/1
and the trunk of Gig 0/0.
And then on Switch-2, we've got the two VLAN 100 entries--
one for PC-1, one for PC-4--
reachable off of the trunk on Switch-2.
And on Switch-2 the local port of Gig 1/1
to reach the MAC address of PC-4.
We also dynamically learned a MAC address ending in DE00.
And that's a MAC address over on Switch-1,
on it's trunk interface, that had
generated at least one frame of traffic on its own.
And that's why it's showing up here.
It's not related to our VLAN 100 traffic.
But indeed, this MAC address is showing up,
and that's the reason why.
In this Nugget, we've done an additional scenario
where we generate some traffic over a switched, trunked
network, made predictions on the MAC address table that
would be on both switches, and then verified that
by implementing that traffic in our lab scenario.
I hope this has been informative for you,
and I'd like to thank you for viewing.