VLAN Trunking Protocol

VTP (Cisco-proprietary) distributes the VLAN database over trunk links: you create a VLAN once on a server and every switch in the same domain learns it. Only the VLAN list is shared; port assignments stay local.

flowchart LR
  server["SW1 · server<br/>creates VLANs"] -->|"revision 12"| client1["SW2 · client<br/>applies"]
  client1 --> transparent["SW3 · transparent<br/>ignores, forwards"]
  transparent --> client2["SW4 · client<br/>applies"]

Modes

Mode	Create VLANs locally	Applies updates	Forwards updates
server	✅	✅	✅
client		✅	✅
transparent	✅ (local only)		✅
off (v3)	✅ (local only)

Configuration

Switch# configure terminal
Switch(config)# vtp domain lab
Switch(config)# vtp version 3
Switch(config)# vtp mode server
Switch(config)# vtp password secret
Switch(config)# end

Domain, version and password must match on all switches. Updates only travel over trunks.

Danger

Every change increases the configuration revision, and switches always adopt the database with the highest revision. Plugging in a used switch with the same domain and a higher revision can replace, and delete, the VLANs of the whole network. Before connecting a switch, reset its revision to 0 by setting it to transparent and back, or by changing the domain name.

VTP version 3 reduces this risk: only the switch promoted with vtp primary (privileged EXEC) can change the database.

Verification

Command	Shows
show vtp status	Mode, domain, version and the configuration revision.
show vtp password	The configured password.
show vtp counters	Sent and received advertisements, useful when updates don’t arrive.