VLANs not showing in configuration

I was asked to hunt down an issue where newly-created VLANs were not showing up in the running configuration (or the startup configuration) of the switch.

lab3850-sw-1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
lab3850-sw-1(config)#vlan 2
lab3850-sw-1(config-vlan)#name test

lab3850-sw-1#sh run vlan 2
Building configuration...

Current configuration:
end

At first, I thought it was a corrupt VLAN database. To test, I removed the vlan.dat file and then recreated it (by adding a VLAN). The problem persisted.

The issue turned out to be that the switch had VTP configured. The standard configuration is to have VTP disabled, but this one switch was missing that statement. VTP is enabled, in “Server” mode, by default.

lab3850-sw-1#sh vtp status
VTP Version capable             : 1 to 3
VTP version running             : 1
VTP Domain Name                 : NULL
VTP Pruning Mode                : Disabled
VTP Traps Generation            : Disabled
Device ID                       : 0c27.2497.ef00
Configuration last modified by 10.12.254.1 at 9-29-21 01:48:41
Local updater ID is 10.12.254.1 on interface Vl12 (lowest numbered VLAN interface found)

Feature VLAN:
--------------
VTP Operating Mode                : Server
Maximum VLANs supported locally   : 1005
Number of existing VLANs          : 15
Configuration Revision            : 0
MD5 digest                        : 0xF3 0x2C 0x87 0xBA 0x73 0xB9 0x79 0xE9
                                    0x72 0x10 0xAE 0x51 0x48 0xFD 0xB4 0xC4

Disabling VTP, by setting it to “transparent” mode, fixed the issue (and brought the configuration in line with the standard.

lab3850-sw-1#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
lab3850-sw-1(config)#vtp mode transparent
Setting device to VTP Transparent mode for VLANS.

lab3850-sw-1#sh vtp status
VTP Version capable             : 1 to 3
VTP version running             : 1
VTP Domain Name                 : NULL
VTP Pruning Mode                : Disabled
VTP Traps Generation            : Disabled
Device ID                       : 0c27.2497.ef00
Configuration last modified by 10.12.254.1 at 9-29-21 01:48:41

Feature VLAN:
--------------
VTP Operating Mode                : Transparent
Maximum VLANs supported locally   : 1005
Number of existing VLANs          : 15
Configuration Revision            : 0
MD5 digest                        : 0xF3 0x2C 0x87 0xBA 0x73 0xB9 0x79 0xE9
                                    0x72 0x10 0xAE 0x51 0x48 0xFD 0xB4 0xC4
lab3850-sw-1#sh run vlan 2
Building configuration...

Current configuration:
!
vlan 2
 name test
end