- VTP (VLAN Trunking Protocol) is a layer 2 protocol that maintains VLAN configurations, managing addition, deletions and changes of VLANs within a VTP domain.
- A VTP domain is one switch or multiple connected switches (via a trunk link) that share the same VTP configuration.
- Only 1 VTP Domain supported per switch.
- VTP domain is “null” by default
- The default VTP mode of a switch is SERVER, but Cisco switches do NOT propagate VTP information out trunk interfaces until a management domain name is specified or learned.
VTP has 3 modes: Server, Client and Transparent
Creates, modifies and deleted VLANS
Sends and forwards advertisements to other switches
Synchronises VLAN configuration with latest information received from other switches in management domain
Saves VLAN configuration in NVRAM
Cannot create, change or delete VLANs (via the CLI)
Forwards advertisements to other switches (does not initiate/send advertisement)
Synchronises VLAN configuration with latest information received from other switches in the management domain
DOES NOT save VLAN configuration in NVRAM (unlike switch operating in server mode)
Maintains its own VLAN database (can create, modify and delete VLANs BUT on the local switch ONLY)
Will neither advertise or accept VLAN database information from other switches
Forwards VTP advertisements received from other switches
VTP Message Types
Summary Advertisements: communicates with adjacent switches the VTP domain name and configuration revision number. The VTP server increments the configuration revision number when VLANs are added, deleted or modified and issues the Summary Advertisements
Subset Advertisements: follows the Summary Advertisement and contains a list of VLAN information
- Every time a VTP server modifies its VLAN information the configuration number increments by 1, it then sends VTP advertisement with the revision number.
- A VTP switch will synchronise an update if the revision number is higher than the current database.
- Cisco switches transmit VTP summary advertisements using a layer 2 multicast frame every 300 seconds (5 minutes) or anytime a change to the VLAN database occurs.
- VTP Updates are sent across the management VLAN (using VLAN 1)
- VTP propagates configuration changes only across trunk links
- 3 versions (1,2 and 3)
- Versions are NOT interoperable
Version 2: Token ring support, Unrecognised TLV, Version independent transparent mode, Consistency checks
Version 3: Support for extended VLANs (1025 – 4094), Support for creation and advertising Private VLANs, Improved server authentication, Protection of incorrect database accidentally coonected to VTP domain, Configurable on a per-port basis
- Prevents unnecessary VLAN broadcasts or multicast traffic flooding a trunk link
- VTP Pruning blocks data frames for a VLAN on the trunk link if no ports are associated to the VLAN on the access switch. When an access port is later assigned to a VLAN that was originally pruned, the access switch will communicate with the VTP server instructing it that it now requires the frames for that VLAN. The VTP server will now start forwarding frames for that VLAN.
- VTP Pruning is disabled by default
Can only be enabled on the VTP Server
Configure the switch as a VTP Server
switch (config)# vtp mode server
Configure domain name
switch (config)# vtp domain LAB
Change VTP version
switch (config)# vtp version 2
Specify VTP Password
switch (config)# vtp password CISCO123
Optional (VTP Pruning)
switch (config)# vtp pruning
switch (config)# interface gigabitethernet 0/1
switch (config)# switchport trunk pruning
Show commands (VTP status)
switch# show vtp status
There are some dangers running VTP. Connecting a switch that is already pre-configured with VLANs and VTP configured into a network could override the existing VLAN database by adding or worse removing existing VLANs, thus causing a major outage. Advise would be to configure switches in transparent mode “vtp mode transparent” and manually add VLANS as needed. If VTP is used ensure proper procedures are followed when connecting additional switches to the network.