We all know
that the role of the VTP protocol is to replicate the VLAN information to all
switches in a VTP domain. So when the network administrator creates new VLAN
(say VLAN100) on one switch, the VTP will automatically update all the switches
in the network.
VTP stands
for VLAN Trunking Protocol, and comes in three modes:
1- Server Mode: the
switch that’s configured in this mode will allow VLANs to be created and/or
deleted. VLANs are stored in VLAN.dat file under the flash directory. VTP will send the updated information to all
other switches in the LAN.
2- Client Mode: the
switch that’s configured in this mode will not allow any VLAN creation or
deletion. All the VLAN information is taken from the VTP server.
3- Transparent
Mode: just like the VTP server mode, it allows VLAN creation and/or deletion,
and store VLAN information in VLAN.dat. But unlike server mode, transparent
mode will not tell any switch any VLAN information, and will not listen to any
updates from any other switch.
There’s a
debate out there about where a VTP Client stores its VLAN information. Whether
it’s stored in a VLAN.dat file (just like a VTP server), or just taken from the
VTP server, even after a reboot.
As a
reminder, VLANs stored in VLAN.dat file will save them from being erased, even
after we “write erase” our switch.
Let’s try it
out on Packet tracer!
The below
illustration is all what we need to test our understanding
Here’s the
basic configuration:
VTP_CLIENT(config-if)#int
fas0/1
VTP_CLIENT(config-if)#switchport
mode trunk
VTP_CLIENT(config)#vtp mode client
VTP_CLIENT(confi#vtp domain VTP
|
VTP_SERVER(config)#int fas0/1
VTP_SERVER(config-if)#switchport
mode trunk
VTP_SERVER(config)#vtp mode server
VTP_SERVER(config)#vtp domain
VTP
|
Now let’s
update our VTP server switch by adding a new VLAN (VLAN100). As you might
guess, VTP will update the VTP Client with this information. To make sure, just
do “show vlan brief” on VTP client!
VTP_CLIENT#
show vlan brief
VLAN
Name
Status Ports
----
-------------------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/3, Fa0/4, Fa0/5
Fa0/6, Fa0/7, Fa0/8, Fa0/9
Fa0/10, Fa0/11, Fa0/12, Fa0/13
Fa0/14, Fa0/15, Fa0/16, Fa0/17
Fa0/18, Fa0/19, Fa0/20, Fa0/21
Fa0/22, Fa0/23, Fa0/24
100
VLAN0100
active
|
VTP_SERVER(config)# vlan 100
|
Now, for us
to make sure that even when the switch is in VTP Client mode, it will keep the
VLAN information, we’ll do the following test:
1- Disconnect the
link between the two switches. This will ensure that the VTP Client will not
update itself from the VTP server
2- Issue “write
erase” on the VTP client
3- Issue reload on
VTP client
After the
switch boots up, we can notice that the switch name is returned to the default
name, i.e. “switch”, which tells that the switch is returned to its factory
default.
Let’s issue
“show vlan brief” again on the client switch
Switch#show vlan brief
VLAN Name Status Ports
----
-------------------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/15, Fa0/16
Fa0/17, Fa0/18, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Fa0/24
100 VLAN0100 active
|
Sure enough!
VLAN100 is still there. This is a good proof that VTP client saves its information
in the VLAN.dat, the fact that can easily be hidden from many.
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