What is VTP?

Cisco proprietary protocol to distribute VLAN database (IDs and names) across switches.
Reduces admin effort by syncing VLANs via VTP advertisements over trunk links.

VTP Modes

Mode	Function
Server	Create, modify, delete VLANs; updates sent to others; stores in NVRAM.
Client	Cannot create VLANs; receives and applies VLAN updates.
Transparent	Does not participate in VTP updates; forwards them (v2+); VLANs local only.

VTP Message Types

Message Type	Purpose
Summary Advertisement	Sent every 5 min or on change. Includes domain, revision number, etc.
Subset Advertisement	Carries actual VLAN data after change. Follows summary ad.
Advertisement Request	Sent by switch when it needs VLAN info (e.g., just rebooted).
Join Message (v3)	Used for authentication and VTPv3-specific behavior.

Summary Advertisement Packet Breakdown

Field	Value
Source MAC	Switch's MAC (e.g., aabb.cc80.0300)
Dest MAC	01:00:0C:CC:CC:CC (Cisco multicast)
VTP Domain	Configured name (must match for sync)
Revision Number	Indicates latest config
Version	1, 2, or 3
Type	Summary Advertisement

VTP Election

Automatic. No manual configuration needed.
No “election” like STP — VTP uses highest revision number to apply config updates.
Switch with higher revision number wins (careful! even a blank client with high revision can wipe your VLANs).

Configuration Revision Number

Automatically increments when a change is made to VLANs.
Cannot be set manually.
Best practice: reset it to 0 before joining a new switch using delete vlan.dat and reload.

VTP Versions

Version	Features
VTPv1	Basic VLAN sync.
VTPv2	Adds Token Ring support, Transparent-to-Transparent updates, and consistency checks.
VTPv3	Supports extended VLANs, Private VLANs, and VTP off mode. Password mandatory for sync.

Preferred:

VTPv2 if using standard VLANs only.
VTPv3 if using extended VLANs or Private VLANs.

VTPv2 Consistency Checks

Transparent switches validate VLAN info before forwarding.
Prevents malformed VLAN updates from spreading (e.g., invalid VLAN ID).
Protects VLAN integrity across domain.

Best Practices

Use VTP Transparent Mode in production unless VLAN sync is critical.
Set domain name and password manually.
Always clear VLAN database before adding new switches (delete vlan.dat).
Use pruning to restrict unnecessary VLANs on trunks (enabled on server, honored by clients).

VTP - Summary Advertisements:
Summary advertisements are sent every 5 minutes by default. They inform switches about the current VTP domain name and configuration revision number. This helps ensure all switches in the domain have the same VLAN information. If a switch receives a summary advertisement with a higher configuration revision number, it requests the full VLAN database to update its own records.

Discover the Basics of VLAN Trunking Protocol (VTP)

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