- [Instructor] In this video, we want to review
the operation of IEEE 802.1Q trunking,
or we typically say just Dot1Q trunking for short.
The idea is we might have switches like we see here
with ports in different VLANs
and each of these two switches have clients
belonging to VLANs 10, 20, and 30.
How do we pass traffic from the VLAN 10 computer
on the top switch to the VLAN 10 computer
on the bottom switch?
Well, I suppose we could have a link
to interconnect the switches
that just carry traffic for VLAN 10
and we can have another physical link
that just carried traffic for VLAN 20 and so on,
but you can see that's not scalable at all.
What we can do instead is create a special type
of connection between these switches called an 802.1Q trunk.
And this 802.1Q trunk has a very unique ability.
It is able to carry traffic for multiple VLANs
over the same physical link.
So here, traffic for VLANs 10, 20, 30,
they're all flowing over that Dot1Q trunk.
And the way the destination switch is able
to know where to send that traffic is
it looks at the VLAN identifier contained
in one of the tag bytes that get added onto this trunk.
We add on four tag bytes.
There is one exception.
There is a special VLAN, and by default
it's VLAN 1 on our Cisco Catalyst switches,
but we call that special VLAN a native VLAN.
What's unique about a native VLAN is
that it doesn't get tagged.
We don't add four bytes to that native VLAN,
and that means if a switch receives
a frame coming in over a trunk and it has no identifier,
it doesn't have those four extra bytes added.
Then it's going to assume it belongs to the native VLAN,
and it knows it should only send it out
a port belonging to the native VLAN on that switch.
And as a result, it's very important
that both switches agree that for this trunk,
they share the same native VLAN;
otherwise, we could do what's called VLAN hoping
and go from one VLAN on one switch to another VLAN
on another switch because we aren't
explicitly marking a native VLAN.
It's just that the absence of a marking implies
the VLAN membership.
Now, let's take a closer look at
the frame format of a Dot1Q trunk.
Here we see a traditional ethernet frame,
but with a Dot1Q trunk we insert these four bytes
that we see here between the source address
and the type field in an ethernet frame,
and we do that again for every VLAN except
the native VLAN,
and there are VLAN bits in amongst these four bytes
that indicate VLAN membership.
And when we're doing trunking between switches,
802.1Q is what we typically use.
That's an industry standard.
You may be working on some older configurations however.
Just to be aware of this, you might run into an ISL trunk.
That's an interswitch link trunk.
That's a Cisco propriety way of doing trunking
between Cisco Catalyst switches
and it's really not what's preferred these days.
It's got a large header.
It's got 30 bytes that we add on.
There's 26 bytes of header and four bytes of checksum.
That's a lot to be adding to our frames.
And because it is a Cisco tagging mechanism,
it might not work well with other vendors.
So in almost all cases today
we're going to be using Dot1Q trunking.
Now, this was a review of Dot1Q trunking operation.
In our next video, we're going to take a look at
some common troubleshooting issues
that we might run into with Dot1Q trunks.