Configuring VLANs on one or two switches is easy enough, but as the scope of the VLANs increases managing each switch's VLANs individually can become an administrative headache, and prone to mistakes and inconsistencies. Therefore a VLAN management protocol can make this much, much easier.
VTP is one of a couple of VLAN management protocols, the others being GARP VLAN Registration Protocol (GVRP) these are defined in 802.1D (GARP) and 802.1Q (clause 11) for GVRP. These others are not currently supported by Cisco and are therefore not part of the Switch exam, or the CCIE exam.
VTP manages the addition, deletion and renaming of VLANs across the network from a centralised point.
VTP uses domains (areas with common VLAN requirements). A switch can only belong to one VTP domain.
To participate in a VTP domain each switch must be in one of the following modes:
1. Server mode - this has full control over VLAN creation, deletion and modification, this is the default mode for switches.
2. Client mode - cannot create, change or delete and VLANs. Clients listen to VTP advertisements from other switches and modify their own VLAN configuration. Also acts as a relay, forwarding VTP information to other switches.
3. Transparent mode - does not participate in VTP, and does not advertise its own VLAN configuration. With VTP version 1 it does not relay VTP information from other switches unless the VTP domain names and numbers match. In VTP version 2 they can still act as relays regardless of the VTP domain name.
VTP advertisements are sent using multicast and reference an index called the configuration revision number. The revision number starts at zero and increments by one with any change, at which point the update is sent to all VTP listening switches.
New switches should always be cleared of all VTP information before being introduced into the network.
VTP configuration
Head back into the PacketTracer lab we created earlier and add the following:
Management domain:
switch0(config)# vtp domain vtptest
switch1(config)# vtp domain vtptest
Mode:
switch0(config)# vtp mode server
switch0(config)# vtp version 2
switch0(config)# vtp password vtppassword
switch1(config)# vtp mode client
switch1(config)# vtp version 2
switch1(config)# vtp password vtppassword
Now we can add a couple of VLANs - so on Switch0 add VLANs 4 and 5, then do a sh vlan on Switch1
Without adding anything to Switch1 you should see
Running a show vtp status on switch0 you should see
VTP Pruning
VTP Pruning makes more efficient use of trunk bandwidth by reducing unnecessary flooded traffic.
To implement this it is just a case of typing "vtp pruning" in config mode. But this is not supported in PacketTracer, at least not in the version I am running. It might be supported in future releases.
switch(config)# vtp pruning
Once this is added on server it is also enabled on the VTP clients.
Troubleshooting VTP
Check the mode if updates are not being received - make sure not in transparent.
Makes sure at least one switch is set as the server.
Make sure trunks are used as VTP advertisement are only sent over trunk links.
Make sure VTP domain name is identical across all switches.
VTP versions should be compatible.
Passwords should match for VTP.