VLAN Trunking Protocol (VTP) Overview
Learn how Cisco VTP synchronizes VLAN database information, how VTP domains and advertisements work, and why VTP is different from VLAN trunking.
VLAN Trunking Protocol (VTP) is a Cisco-proprietary switch messaging protocol. It exchanges and synchronizes VLAN database information among compatible Cisco switches. VTP can distribute information such as VLAN IDs and VLAN names within a configured VTP domain.
Despite its name, VTP is not the protocol that makes a link carry traffic for multiple VLANs. VLAN trunking uses a trunking technology such as IEEE 802.1Q. VTP uses connectivity between switches to exchange VLAN information.
Prerequisites
Before studying VTP, understand the purpose of a computer network, basic Ethernet switching, VLAN IDs, switch-to-switch links, and the basic purpose of VLAN trunks. A VLAN is a logical Layer 2 network segment identified by a VLAN ID and commonly assigned a VLAN name.
What VTP Does
VTP allows participating switches to share a VLAN database. The database contains information about VLANs that switches can use, including VLAN IDs and names. When a VLAN change is made on a switch, VTP can advertise that change to other compatible switches in the same VTP domain.
Common database changes include:
- Adding a VLAN.
- Deleting a VLAN.
- Changing VLAN information, such as a VLAN name.
A VTP advertisement is a VTP message that communicates VLAN database information or a VLAN change between switches.
Administrative Benefit of VLAN Synchronization
Without a VLAN synchronization mechanism, an administrator may need to create the same VLAN manually on every switch that requires it. This creates repetitive work and increases the chance of inconsistent VLAN IDs, names, or availability.
For example, consider an organization with 100 switches. If a new VLAN is needed throughout the network, an administrator working without VTP may have to enter the VLAN configuration separately on each relevant switch. Missing one switch can cause connected devices to have different VLAN availability from the rest of the network.
With VTP, the administrator can create the VLAN on one participating switch. The VLAN addition can then be advertised throughout the shared VTP domain. Deletions can also be distributed, which makes centralized changes efficient but potentially high-impact.
VTP Domains
A VTP domain is a logical group of interconnected switches configured to share compatible VTP VLAN information. Switches must use matching VTP domain settings to participate together.
The main compatibility settings to recognize are:
- VTP domain name: The configured domain identifier. It must match on switches intended to exchange VTP information as members of the same domain.
- VTP password: An optional authentication setting used to protect VTP advertisements. When configured, participating switches must use the matching password.
- VTP version: The protocol version selected for compatible operation among domain members. Switches must use compatible versions.
- Operating mode: The selected VTP role, such as server, client, or transparent. Mode selection is a follow-on configuration subject because it determines how a switch handles VLAN information.
These settings do not replace the need for physical or logical connectivity. The switches must also be connected through operational trunk links so that VTP advertisements can travel between them.
How VTP Advertisements Propagate
Suppose three switches have this topology:
SW1 ===== trunk ===== SW2 ===== trunk ===== SW3All three switches belong to the same VTP domain and have compatible settings. The propagation sequence is:
- An administrator creates a new VLAN on SW1.
- SW1 generates VTP information describing the VLAN database change.
- SW1 sends the VTP update across its operational trunk to SW2.
- SW2 receives the compatible update and forwards the information through its other trunk toward SW3.
- SW3 receives the update and obtains the corresponding VLAN information.
The intermediate switch, SW2, allows the information to continue through the switched network. When propagation succeeds, all compatible participating switches in the domain have the VLAN information.
The same principle applies to deletion. If an existing VLAN is deleted on one participating switch, the deletion can be advertised to other switches in the same VTP domain. This is why centralized VLAN changes require careful operational control.
VTP Is Not a Trunking Protocol
The word trunking in VTP's name can cause confusion. VTP synchronizes VLAN database information; it does not make a switch-to-switch link carry frames from multiple VLANs.
A trunk link is a switch-to-switch link that carries traffic for multiple VLANs. The trunk uses a VLAN tagging or encapsulation method to identify the VLAN associated with traffic. VTP uses the connectivity provided by the trunk to transport its advertisements, but VTP does not configure or provide that trunk.
| Concept | Primary purpose | What it carries or changes | Examples |
|---|---|---|---|
| VTP | Synchronize VLAN database information between compatible Cisco switches. | VLAN information and changes, such as VLAN IDs and VLAN names. | VTP advertisements within a VTP domain. |
| 802.1Q trunking | Carry traffic for multiple VLANs over one Ethernet link. | Ethernet frames identified with IEEE 802.1Q VLAN tags. | IEEE 802.1Q, the standard VLAN tagging method. |
| ISL trunking | Carry traffic for multiple VLANs using a Cisco trunk encapsulation method. | Ethernet frames encapsulated using Cisco Inter-Switch Link. | ISL, a legacy Cisco trunking method. |
In short, 802.1Q and ISL provide the trunking mechanism, while VTP exchanges VLAN database information.
VTP Domain Participation Requirements
| Requirement | Why it matters |
|---|---|
| Trunked switch connectivity | VTP advertisements need an operational path between switches. A trunk link transports the advertisements. |
| Matching VTP domain name | Switches use the domain name to identify the group with which they are intended to share VLAN information. |
| Matching VTP password when configured | Authentication must succeed before switches can exchange protected VTP information. |
| Compatible VTP version | Switches must support and use compatible protocol behavior to process one another's advertisements. |
Conceptual Configuration Values
Detailed command syntax is a separate configuration exercise. For an overview, recognize that a VTP design requires decisions about:
- The VTP domain name.
- Whether a VTP password is used.
- The VTP version.
- The operating mode of each switch, such as server, client, or transparent.
- Whether the inter-switch links are operational trunks, normally using IEEE 802.1Q on supported platforms.
VTP mode selection should be studied alongside VLAN creation and management. The mode affects whether a switch can originate, learn, or relay VLAN database information.
Troubleshooting VTP
A VLAN does not appear on another switch
If a VLAN created on one switch does not appear on another switch, check the complete path rather than assuming that VTP itself is the only issue.
- Confirm that the switches are connected through working trunk links.
- Verify that the switches use the same VTP domain name.
- Verify the VTP password when authentication is in use.
- Verify that the VTP versions are compatible.
- Confirm that the receiving switch is configured to participate in the intended VTP domain and operating mode.
- Check each intermediate switch in the path. An incompatible or incorrectly configured intermediate switch can prevent the update from reaching later switches.
An administrator expects VTP to create a trunk
VTP does not configure trunking. Check the actual switch-port configuration and operational state separately. Configure and verify the appropriate trunking method, such as IEEE 802.1Q, before expecting VTP advertisements to travel.
Connection to Later Switching and Routing Topics
VTP explains how VLAN information can become available across multiple switches, but it does not by itself carry user traffic between VLANs. After VLAN availability and trunking are understood, study router-on-a-stick inter-VLAN routing to learn how one router interface can use a trunk to route between VLANs.
Useful follow-on subjects include VTP modes, VTP versions, VLAN creation on Cisco switches, IEEE 802.1Q trunk configuration, legacy ISL, and VLAN troubleshooting. For broader Layer 2 and Layer 3 context, review the OSI reference model.
Exam-Relevant Summary
- VTP is a Cisco-proprietary switch messaging protocol.
- VTP exchanges and synchronizes VLAN database information among compatible Cisco switches.
- VLAN IDs and VLAN names are examples of information that can be distributed.
- A VTP domain is a group of switches configured to share compatible VTP information.
- Matching domain names, passwords when configured, and compatible versions are required for participation.
- Operational trunk links provide the path for VTP advertisements.
- Intermediate switches can forward VTP information to additional compatible switches.
- VTP is not VLAN trunking.
- IEEE 802.1Q and Cisco ISL are trunk encapsulation technologies, not VTP.
- VTP can distribute VLAN additions and deletions, so centralized changes must be handled carefully.