VTP vs. DTP: Key Differences Explained

This article compares VTP (VLAN Trunking Protocol) and DTP (Dynamic Trunking Protocol), highlighting the differences between them.

VTP (VLAN Trunking Protocol)

Function:

VTP, or VLAN Trunking Protocol, is used to propagate VLAN definitions across an entire Local Area Network (LAN). To achieve this, VTP carries VLAN information to all switches within the VTP domain. In VTP mode, VLAN packets are transmitted using ISL (Inter-Switch Link) frames or 802.1Q compliant frames.

A trunk is a physical and logical connection between two switches that allows network traffic to travel between them. In a VLAN context, a trunk functions as a conduit for VLANs between switches and routers, and between switches themselves.

• VTP Domain: Refers to one or more interconnected switches that share VLAN configuration details.

• VTP Advertisements: Used to distribute and synchronize VLAN configuration across the VTP domain.

VTP Modes

A switch can be configured in one of three VTP modes:

• VTP Server:
  • Advertises VLAN information across the domain.
  • Allows you to create, delete, and modify VLANs.
• VTP Client:
  • Stores VLAN information received from the server.
  • Does not allow you to create, delete, or modify VLANs.
• VTP Transparent:
  • Forwards VTP advertisements.
  • Does not participate in VTP (doesn’t learn or advertise VLANs).

Features of VTP

• VTP operates between switches.
• VLAN configuration is typically done on one switch (the switch in VTP server mode).
• VTP automatically propagates VLAN information to all switches in the domain (switches in VTP client mode).

Example:

Imagine an office building with six floors, each having a switch configured with VLANs. If you need to add a new department to the network, without VTP, the system administrator would need to manually configure the new department’s VLAN information on each switch on all six floors.

While manageable for smaller setups, this becomes cumbersome and time-consuming in larger environments. VTP simplifies this process. The administrator only needs to configure the master switch (the switch in VTP server mode), and the rest of the switches automatically receive the updates through VTP advertisements.

Advantages of VTP

• Consistent VLAN Configuration: Ensures VLAN configuration is consistent across the entire VTP domain.
• Easy Trunking: Simplifies VLAN trunking over mixed media (e.g., mapping Ethernet VLANs to high-speed ATM LANE or FDDI VLANs).
• Accurate Monitoring and Control: Allows for accurate monitoring and control of VLANs.
• Dynamic Reporting: Enables dynamic reporting of added VLANs in the network.
• Simplified Addition of New VLANs: New VLAN additions follow a simple “Plug and Play” configuration.

DTP (Dynamic Trunking Protocol)

DTP, or Dynamic Trunking Protocol, is another important concept in networking.

Features of DTP

• Cisco Proprietary: DTP is a Cisco proprietary protocol. Switches from other vendors typically do not support it.
• Automatic Enablement: DTP is automatically enabled on a switch port when certain trunking modes are configured.
• Trunk Negotiation: DTP manages trunk negotiation only if the port on the other switch is configured in a trunk mode that supports DTP.
• Supports ISL and 802.1Q: DTP supports both ISL and 802.1Q trunking protocols.
• Older Hardware Limitations: Some older Cisco switches and routers may not support DTP.

DTP Modes

DTP supports the following modes:

• On (default): (switchport mode trunk) - Periodically sends DTP advertisements to the remote port, dynamically changing it to a trunking state.

• Dynamic Auto: (switchport mode dynamic auto) - The switch port periodically sends DTP frames to the remote port, advertising its ability to trunk, but it doesn’t request to enter the trunking state. It will become a trunk if the other side requests it.

• Dynamic Desirable: (switchport mode dynamic desirable) - DTP frames are sent periodically to the remote port, advertising the port’s ability to trunk and requesting the remote switch port to go into the trunking state.

• Turn off DTP: (switchport nonegotiate) - The local port does not send out DTP frames to the remote port. The local port is then considered to be in an unconditional trunking state. This is often used for security purposes.