Where, oh where has the VLAN information gone? VTP and how it impacts the VLAN Database.

Where, oh where has the VLAN information gone? VTP and how it impacts the VLAN Database.

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This is a topic we learn in the first few days of learning about Cisco.  However, in the normal day to day life we rarely have to think about this information.

We usually do remember the vlan.dat file is stored in the nvram.  What we don’t remember is what kinds of impact the VTP version and mode have on how the switch handles VLAN information.

If you have not watched the video by JP who used to be in IP Expert I recommend watching the video at the end.  Additionally I am posting a link to one of the many places to find configuration for VTP here

https://www.cisco.com/en/US/docs/switches/lan/catalyst3850/software/release/3.2_0_se/multibook/configuration_guide/b_consolidated_config_guide_3850_chapter_0100000.html

Unfortunately they change these paths ALL the time so its best to just do a search using the words “Cisco VTP Configuration 3750” or something similar to that.  You can find the commands in the Master Command reference but that usually does not have the additional information in it.

VTP is what determines where the VLAN information is actually stored on Cisco switches.

So some major points to note.

Topic 1 –  Extended VLANs

In VTP Version 1 and 2 vlan’s above 1004 are NOT stored in the VLAN.dat file.  This was modified in VTP version 3.  In VTP version 1 or 2, if you create a vlan like vlan 1010 it will show up in the running configuration no matter what VTP Mode you are in.

Topic 2 –  Server and Client Mode

When the switch is in VTP Server or Client mode and the VTP version is 1 or 2, the Vlan information such as name and number are stored in the vlan.dat file (assuming it is a standard VLAN and not extended) and NOT in the running configuration.

Topic 3 – Transparent Mode

When the switch is in VTP Transparent mode the vlan information will be stored both in the running configuration AND the vlan.dat file.

Topic 4 – Startup

When the switch boots up it looks for what VTP mode it is set too.  This is critical because if you change the switch to VTP Transparent mode and do NOT save this to the startup configuration.  Upon startup, we could have issues as the switch will default to normal Server behavior and will not load the VLAN information from running configuration.

Topic 5 –  Possible request

You could be asked to ensure that VLAN’s for Switch 10 SVI’s must be in the running configuration, but the VLAN’s that connect it to R2 must NOT be in the running configuration.  In this scenario the SVI’s are VLAN’s 1050, 1070, and 1080.  The VLAN for R2 is 20.

The solution for this would be to ensure that the device is either in VTP Server or Client mode and Version 1 or 2.  You would need to read the other requirements to determine which version and which mode.

Another possible asks could be to have all VLAN’s be in the running configuration.  The solution to this would be to have the switch operating in VTP Transparent mode.

Topic 6- Good commands to know

show vlan brief (this will show you just the vlan data and not the extra settings not normally needed)

vtp file [name]  (this changes the vlan database file name.  In the below outputs you can see the name change)

 

Outputs

SW1#config t
Enter configuration commands, one per line. End with CNTL/Z.
SW1(config)#vtp file NOTWHATYOUARELOOKINGFOR
Setting device to store VLAN database at filename NOTWHATYOUARELOOKINGFOR.

In the running configuration you can see that this has been modified

SW1# show run

****output omitted***

no ipv6 cef
vtp file NOTWHATYOUARELOOKINGFOR                                 ****THIS IS WHERE VLAN’s would be stored*****
!
!

****output omitted***

 

From the Documentation this is the wording for how the switch handles the vlan database file on startup.

******IMPORTANT******
******IMPORTANT******
******IMPORTANT******
When you save VTP information in the switch startup configuration file and reboot the switch, the switch configuration is selected as follows:

******If the VTP mode is transparent in the startup configuration and the VLAN database and the VTP domain name from the VLAN database matches that in the startup configuration file, the VLAN database is ignored (cleared), and the VTP and VLAN configurations in the startup configuration file are used. The VLAN database revision number remains unchanged in the VLAN database.

******If the VTP mode or domain name in the startup configuration do not match the VLAN database, the domain name and VTP mode and configuration for VLAN IDs 1 to 1005 use the VLAN database information.
******IMPORTANT******
******IMPORTANT******
******IMPORTANT******

 

 

If you like JP Cedeno’s teaching style while the IP Experts company no longer exists, he creates content both on his own page and using udemy.

Link to JP’s Udemy Page https://www.udemy.com/user/jp-cedeno/

Link to JP’s Main Page https://www.xtremeie.com/about.html

 

 

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