VLAN Trunking Protocol (VTP ) is a Cisco proprietary protocol used to provide consistency to the VLAN configuration over the network by providing centralised control over the VLAN domain. It operates in three modes – server, client, and transparent. By default, VTP version 1 is enabled. Before configuring a VTP client we have to ensure that its VTP revision number is lower than the VTP revision number of the network otherwise the client will overwrite the VTP configurations on the entire network.
The major difference between VTP version 2 and version 3 is that version 3 provides support for enhanced authentication by letting the user save the password as hidden or encrypted. Moreover, VTP version 2 can be automatically configured when it receives a VTP message, whereas, in version 3 we have to do the configurations manually. Version 3 also extends support to the extended VLANs. VTP Version 1 and 2 supports VLAN 1 to 1004, whereas, version 3 can provide support to VLAN 1006 to 4094, remember that VLANs 1002 to 1005 still remain reserved and cannot be modified and pruning still applies only to VLAN 1 to 1005. Version 3 also provides support for the private VLANs, which is not possible in version 2. VTP version 3 supports the concept of a primary and secondary server. A VTP primary server is used to update or change the domain information and can pass on the updated information to the secondary servers, and a secondary VTP server can only back up to the updates provided by the primary server. By this, it gives better administration control over which devices can make changes to the configuration, and in turn, helps maintain consistency over the network efficiently. It also protects from unintended database overrides when new switches are added to the network. All devices come set up as secondary servers by default and we have to run the vtp primary command in the privileged configuration mode. Another major difference between version 2 and 3 of the VTP is that we can turn-on or turn-off VTP on every port (or trunk) independently, by using no vtp interface configuration mode command. Version 3 also supports the propagation of databases other than the VLAN database, for instance, the MST database.
To conclude, we can say that version 3 of VTP is just an extension of version 2, and by using version 3 we can make a VLAN more secure, and also make the administration of the VLAN more efficient.
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