Inside VTP: Understanding Advertisements, Message Types, and Revision Numbers
Introduction
In VLAN Trunking Protocol (VTP), switches don’t work in isolation – they constantly talk to each other to make sure all VLAN information stays in sync.
This coordination happens through VTP Advertisements – special messages that carry VLAN data between switches.
However, what really decides which switch’s VLAN data wins is a small but critical value called the Configuration Revision Number.
A wrong revision number can wipe out your entire VLAN database in seconds!
In this blog, we’ll explore both sides of the story:
- How VTP Advertisements synchronize VLAN information
- The three message types (Summary, Subset, and Request)
- The importance – and danger – of revision numbers
- Best practices and VTPv3 improvements for safer operations
What Are VTP Advertisements?
VTP Advertisements are messages exchanged between switches that share the same VTP domain.
Their purpose is simple: to ensure all switches have consistent VLAN information – same VLAN IDs, names, and states.
When a VTP Server makes any change in VLAN configuration (add, delete, rename, etc.), it sends out advertisements to all other switches in the VTP domain.
These advertisements travel through trunk links to reach other switches.
How VTP Advertisements Propagate VLAN Information
Here’s how VTP keeps your VLANs synchronized step by step:
- A change occurs on a VTP Server (e.g., VLAN 40 is added).
- The server increments its revision number by one.
- It sends a Summary Advertisement to its neighbors.
- Neighboring switches compare the revision number they have with the one they received.
- If the received number is higher, they request full details using a VTP Request Advertisement.
- The server responds with Subset Advertisements that contain all VLAN details.
- The process continues until all switches have the same revision number and VLAN data.
Analogy:
Think of the VTP server as a “team leader” updating a shared spreadsheet.
Every time the leader edits something, they increase the version number and send a copy to everyone else.
Each team member keeps the latest version — the same logic applies to switches.
Understanding the Configuration Revision Number
The Configuration Revision Number is like a version counter for the VLAN database.
Each time the VLAN configuration changes on a VTP Server:
- The revision number increases by 1.
- That updated number is advertised across the network.
For example:
| Event | Action | New Revision Number |
| Initial State | – | 0 |
| Add VLAN 10 | Change | 1 |
| Rename VLAN 10 | Change | 2 |
| Delete VLAN 10 | Change | 3 |
This number is stored in the VTP database (vlan.dat) file, even after reboot.
Why Incorrect Revision Numbers Can Cause VLAN Loss
This is one of the most common – and most dangerous – VTP mistakes.
If you connect a switch with a higher revision number but an empty VLAN database to your production network, it can overwrite VLAN information on all other switches in the domain.
Result:
All VLANs on other switches get deleted and replaced with the empty VLAN list — leading to loss of network connectivity.
Example:
- Switch A (Production): Revision 5 (with VLANs 10, 20, 30)
- Switch B (Lab switch): Revision 10 (empty VLANs)
- When connected: Switch A will accept the advertisement from B (since 10 > 5)
- VLANs 10, 20, 30 will be deleted from all switches.
Hence, higher revision number = higher priority, even if the data is wrong!
Best Practice: Reset the Revision Number Before Adding a New Switch
Before adding a switch to an existing VTP domain:
- Change its VTP mode to Transparent (resets revision number).
- Change it back to Client or Server.
- Verify revision number is 0 using:
| show vtp status |
- Only then connect it to the production network.
This ensures no old or incorrect VLAN information spreads unintentionally.
VTPv3: Primary and Secondary Server Concept
VTPv3 introduced a safety feature to prevent accidental VLAN overwrites.
In VTPv3:
- Only the Primary Server can make VLAN configuration changes.
- Secondary Servers can advertise and sync but cannot modify VLANs.
This adds a layer of control, ensuring VLAN changes are intentional and authorized.
How to Promote a Switch to Primary:
| Switch(config)# vtp primary vlan |
You’ll then see a confirmation message before it becomes primary.
VTP Advertisement Message Types
VTP uses three types of advertisements — Summary, Subset, and Advertisement Request — each playing a unique role in keeping VLAN databases aligned.
1. Summary Advertisements
Summary Advertisements act as the “hello messages” of VTP.
They are sent every 5 minutes (by default) or whenever there’s a VLAN change.
Each Summary Advertisement includes:
- VTP Version
- VTP Domain Name
- Configuration Revision Number
- MD5 Digest (authentication hash)
- Updater Identity (which switch sent it)
Purpose:
To inform neighboring switches about:
- The current revision number.
- Whether they need to update their VLAN database.
If a neighbor sees a higher revision number in the summary, it immediately requests detailed VLAN info (via a Request Advertisement).
2. Subset Advertisements
Once a switch detects a higher revision number, it sends a VTP Advertisement Request.
In response, the server sends Subset Advertisements, which contain actual VLAN details — such as:
| Field | Description |
| VLAN ID | VLAN Number (e.g., 10, 20, 30) |
| VLAN Name | Human-readable name |
| VLAN Type | Ethernet, Token Ring, FDDI |
| MTU Size | Maximum Transmission Unit |
| Status | Active / Suspended |
Think of it as:
Summary Advertisement = “I have new updates.”
Subset Advertisement = “Here are the full details.”
3. Advertisement Request Messages
When a switch first joins a VTP domain or detects a newer revision number, it sends an Advertisement Request Message to its VTP neighbor.
This message simply says, “Please send me your VLAN database.”
Typical situations when these are used:
- A new switch joins the VTP domain.
- A switch reboots and misses updates.
- A mismatch in revision numbers is detected.
How Synchronization Happens When Revision Number Increases
The synchronization process can be summarized as:
- A change occurs → Revision number increases.
- Server sends Summary Advertisement to neighbors.
- Neighbors compare revision numbers.
- If the neighbor’s revision is lower, it sends an Advertisement Request.
- The server responds with Subset Advertisements containing VLAN details.
- All switches update their VLAN databases to match the new version.
This ensures every switch in the domain shares identical VLAN information.
Additional Important Fields in VTP Advertisements
Let’s look at the key fields common across VTP messages:
| Field | Function |
| Version | Identifies VTP protocol version (1, 2, or 3) |
| Domain Name | Must match on all switches in the same VTP domain |
| Configuration Revision | Indicates the version of VLAN data |
| MD5 Digest | Used for VTP password authentication |
| Updater Identity | IP or MAC of switch that originated the update |
These fields ensure advertisements are authenticated, synchronized, and consistent across the network.
Best Practices
- Always verify the domain name and VTP version before connecting switches.
- Use VTP passwords to prevent unauthorized advertisements.
- Regularly check revision numbers using show vtp status.
- In large networks, consider using VTPv3 for better control and authentication.
Conclusion
VTP Advertisements are the messengers that keep VLAN configurations synchronized across switches, and the Configuration Revision Number decides which data version wins.
Understanding these internal mechanics helps network engineers avoid costly VLAN mismatches and outages.
Always remember:
“A single wrong revision number can do more damage than a wrong VLAN ID.”
Handle VTP with care — and it will handle your VLANs perfectly!