The inability of VTP to deal with extended range VLANs is mostly historical. When Cisco originally introduced its ISL and VTP, the ISL supported VLAN numbers up to around 1000. The VTP was designed with this in mind and so it did not support VLANs with higher numbers.
A couple of years later, the IEEE 802.1Q standard was proposed. The standard called for VLAN numbers up to 4095, and by supporting both ISL and 802.1Q, Cisco realized that there is a range of VLANs from 1006 to 4095 which were not originally supported by ISL nor by VTP. This is the range that is called the extended range. From the 802.1Q viewpoint, it is not extended at all, however, from the older ISL viewpoint, it is a space of VLAN numbers that was not originally covered by it but which had to be ultimately supported by ISL as well.
The ISL VID field in the ISL frame is 15 bits long, so actually it is longer than the VID field in the 802.1Q tag, however, 5 bits were originally unused which limited the number of supported VLANs on an ISL trunk to 1024. By simply using the unused bits, it was quite easy to make ISL trunks carry extended-range VLANs. However, the VTP was obviously more difficult to extend and Cisco probably felt that it is not worth the effort.
Actually, the extended range VLANs are supported by VTP version 3 (however, most switches support only versions 1 and 2). The VTP version 3 is substantially different from its predecessors. You can read more information here:
VTP version 3 is supported for extended vlan range support genrally if you see By default, all Cisco Catalyst switches are configured to be VTP servers. This is suitable for small-scale networks where the size of the VLAN information is small and easily stored in all switches
There are three version of VTP so far. VTP Version 2 (V2) is not much different than VTP Version 1 (V1). The major difference is that VTP V2 introduces the support for Token Ring VLANs. If you are using Token Ring VLANs, you need to enable VTP V2. Otherwise, there is no reason to use VTP V2. VTP version 3 differs from earlier VTP versions in that it does not directly handle VLANs. VTP version 3 is a protocol that is only responsible for distributing a list of opaque databases over an administrative domain. When enabled, VTP version 3 provides the following enhancements to previous VTP versions:
Support for extended VLANs. Support for the creation and advertising of private VLANs. Improved server authentication. Protection from the "wrong" database accidentally being inserted into a VTP domain. Interaction with VTP version 1 and VTP version 2. Provides the ability to be configured on a per-port basis. Provides the ability to propagate the VLAN database andother databases.
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