DTP and VTP

DTP explained here:

• DTP, stands for Dynamic Trunking Protocol, is a cisco proprietary protocol.
• DTP allows ports to be configured for operating trunk mode automatically based on which administrative mode the switchport is on and which administrative or manual mode the switchport that is connected to it is on.
• Interfaces on old switches, switchport mode is dynamic desirable but interfaces on new cisco switches, switch mode is dynamic auto.
• Trunk mode do not do DTP negotiation.
• Dynamic Desirable can form trunk, also actively want to form trunk.
• Dynamic Auto can form trunk but do not actively want to form trunk.
• Access port do not do DTP negotiation.

DTP Administrative Mode Outcomes

If a port is set to Trunk:

• With Trunk → Trunk
• With Dynamic Desirable → Trunk
• With Access → No trunk (X)
• With Dynamic Auto → Trunk

If a port is set to Dynamic Desirable:

• With Trunk → Trunk
• With Dynamic Desirable → Trunk
• With Access → Access
• With Dynamic Auto → Trunk

If a port is set to Access:

• With Trunk → No trunk (X)
• With Dynamic Desirable → Access
• With Access → Access
• With Dynamic Auto → Access

If a port is set to Dynamic Auto:

• With Trunk → Trunk
• With Dynamic Desirable → Trunk
• With Access → Access
• With Dynamic Auto → Access

VTP explained here:

•  VTP, stands for VLAN Trunking Protocol, is used to configure VLAN is a central VTP server and other switches which are VTP client can sync their VTP database with the VTP server. Usually used in a large network. VTP advertisements are only done in trunk ports. VTP has 3 versions: 1, 2 and 3.

• Difference between 1 and 2 is that, 2 supports Token Ring VLAN.
• There are 3 modes a switch can be at if using VTP. These are Server, Client or Transparent.
• By default, switches are in Server mode.
• All switches in same VTP domain name sync with each other. By default, there is no VTP domain name/ NULL domain name.
• In server mode, switches can add, delete or modify VLANs. And in server VTP, information related to VTP (VLAN database) in saved in NVRAM.
• Revision number is something that is 0 at the beginning but with each add, delete or modification of VLANs, it increases by 1.
• All switches with VTP enabled and is synching with the server will change it’s revision number with the one with the maximum number, because the one with the maximum revision number means it is the latest or most recent database.
• VTP clients can not do add, delete or modify. They do not store vlan database in NVRAM in version 1 and 2. In version 3 VTP, switches do store the VTP database in the NVRAM.
• VTP clients sync with VLAN server, who has the highest revision number.
• VTP client will also advertise their own VLAN database to other switches over their trunk port. And they will also pass VTP advertisement they received from any switches in trunk ports, to all the other switches in trunk ports.
• VTP Transparent mode switches, do not update their database, only forwards the VTP advertisement frames to other switches on the trunk port if the VTP domain name matches.
• By changing domain name, revision number becomes 0 again. Also, changing VTP mode to transparent, makes the revision number 0.