- [Instructor] While VLAN's are used
extensively in modern networks,
how are VLAN tagged frames transmitted
from switch to switch?
And the answer is via VLAN trunks.
As discussed earlier, the VLAN protocol
used to tag frames with their VLAN identifiers is 802.1Q.
Once the frame is tagged in a switch,
special ports known as trunk ports
can be configured between switches
that allow them to transmit frames
across the link while maintaining the
VLAN tags on frames.
These VLAN's can spend two switchers or ten switches.
I'll use this diagram for my example.
Both test host ports have been properly configured
and the VLAN's already exist on the switches.
I'll start a protestant ping between hosts
for verification at the trunk between
switches as operating as expected.
On switch one, I'll begin configuration on port three.
Config T, interface fa0/3.
Some switch models require that the
trunking in capsulation be manually set,
while others only support 802.1Q.
My switches require it and this is done
with the switch port trunk in capsulation dot one Q command.
This only sets the encapsulation type,
I now must tell the port to begin trunking with
switch port mode trunk.
This switches configuration is now complete.
I'll move to switch five
and complete the same configuration port on port three.
Config T, interface fa1/0/3,
switch port trunk encapsulation
dot one Q, switch port mode trunk.
I should now be able to pop back over to my test node
and sure enough I see the ping is completing now.
When on trouble shooting VLAN trunking issues,
my go to command to ensure VLAN's are spanning the link is,
show interface fa0/3 the interface in question
and the word trunk.
Taking a look at this last line,
if the VLANing question isn't listed,
then it won't be forwarded.
It's generally down to one of a few things.
Either the VLAN hasn't been created on the switch,
VLAN spanning tree protocol's blocking it,
or it isn't in the VLAN allow list for this port.
If a VLAN is to traverse switch in any way,
it must exist in the VLAN data base.
So an administrator must first create it.
STP will be covered in one of the upcoming chapters.
By default, when a prot is configured to trunk,
it will trunk all VLAN's across the link.
I generally try and limit the number of VLAN's on a link
to the specific ones that belong there.
This is accomplished with the VLAN allow list.
In the example above, it should only be VLAN ten traverse.
So, the commands are config T, interface fa0/3,
switch port, trunk allow VLAN ten.
The VLAN allow list can consist of ranges,
comma separated entries, or a single VLAN.
Now that I've updated the allow list,
the show interface fa0/3 trunk command
should now reflect the fact that only VLAN ten is allowed.
Trunk ports will sometimes have untagged frames into them.
The port must do something with these frames,
so by default they will put them in the native VLAN,
which is VLAN one.
Essentially, the trunk port will act similarly
to an access port in this respect.
The native VLAN can be changed
with the switch port trunk native VLAN command.
Cisco has a proprietary protocol named,
VLAN trunking protocol or VTP,
that was designed to advertise VLAN configuration
information from one switch to another.
Though this protocol can be convenient,
I keep it disabled and in fact the CCNA
and CCNP don't cover it's configuration.
Having said that, it can have some impact
on your configuration so I'll cover the basics.
For older switchers, VTP can't be disabled,
but it can basically be rendered mute,
with the VTP mode transparent global configuration command.
For more modern switches,
the global VTP mode off command,
will completely disable it.
If VTP is neither off nor in transparent mode,
there are several limitations.
First, only the standard VLAN range is available,
one to one thousand five.
When disabled, the extended range will reach up to 4094.
Second, client switches aren't allowed
to configure VLAN's at all,
and last, switches in server and client mode
can learn or have VLAN's deleted
from other VTP speakers.
VLAN's alone are great,
but as soon as you have more than one switch in the mix,
trunks become just as important.