Why a "vlan.dat" file for an IOS switch?

It looks to me the "vlan.dat" design will stay a long life on IOS switches.



Here is the documentation why Cisco added the "vlan.dat" design.

Background Information


VLAN or VTP configuration changes in CatOS are written to NVRAM immediately after a change is made. In contrast, Cisco IOS software does not save configuration changes to NVRAM unless you issue the copy run start command.


VTP client and server systems require VTP updates from other VTP servers to be immediately saved in NVRAM without user intervention. The VTP update requirements are met by the default CatOS operation, but the Cisco IOS software update model requires an alternative update operation. For this, a VLAN database was introduced to Cisco IOS software for Catalyst switches as a method to immediately save VTP updates for VTP clients and servers. This VLAN database is in the form of a separate file in NVRAM called the vlan.dat file.

Reference:

• Cisco.com, Managing vlan.dat in Cisco Catalyst Switches Running Cisco IOS Software