Vlan Trunking Protocol (VTP)

VLAN Trunking Protocol is a Cisco proprietary protocol that propagates the definition of Virtual Local Area Networks on the whole local area network. To do this, VTP carries VLAN information to all the layer 2 switchin a VTP domain.

To update the Vlan or to create, delete the vlan network admin must visit to the every switch, which increase admin overhead so to oversome this VTP is created.

We create Vlan on one switch and other will get updated automatically by the help of VTP message. Basically VTP message is just vlan.dat file containing vlan information which is stored in flash. Once vlan.dat file is received by other switch they will not get updated but will replace previous vlan.dat file with new one.

Let's know few important point:

  • VTP is layer 2 Protocol.
  • It is a Cisco proprietary protocol.
  • Multicast address is 01:00:0C:CC:CC:CC.
  • VTP message is just a Vlan.dat file.
  • VTP has 3 modes i.e Server, Clint and Transparent mode.
  • On all switch VTP domain name must be same.
  • VTP has 4 message types.
  • VTP message are accepted on the basis od VTP domain name.

While creating VTP on switches you have to define modes of VTP according to requirement. Let's know the three (3) different modes os VTP:

Server Mode:
- In server mode we can create vlan, delete vlan and modify vlan.
- Can create Normal range of Vlan i.e. 0-1005.
- Can update all switch whether any other server mode switch or client mode switch.
- It can be updated itself by other server also if exist.
- Server mode is default enable on cisso switches.
Client Mode:
- Client mode switch can not create, delete or modify Vlan on switches.
- These can get updated from Server.
- Client will also forward update coming from server to client or server to other clent or server.
Transparent Mode:
- In Transparent mode switch will never forward its own upadate to other switch and also never upadate itself from other switch.
- In this mode we can create both Normal range and Extended range Vlan.
- Switch will not update itself with others update but still it will forward coming upadates to server or slient. It just passes the information.
- We can craete, delete and modity Vlans on transparent mode.

Now, if you want to spread vlan information within layer 2 network then we use Server and Client mode and do not use Transparent but if we don't want to spread information then we only use Transparent mode and do not mix both.

VTP requirements

- Switch will only forward VTP information through trunk port.
- On all switch VTP domain name must be same.

VTP message types

VTP generate few messages to spread vlan information in layer 2 network. It has 4 types of messages. Note thet these messages are only generate in Server and client mode and Transparent mode switch do not generate any messages.
ð VTP Summary Advertisement
In 5 minute intervals VTP sends Summary advertisement message to its adjencies switch. Included in this VTP Summary Advertisement is the VTP domain name and configuration revision number.
Another VTP Server or Client receives the Summary Advertisement and compares the VTP domain name and revision number to its own. If the domain name is different, it ignores the advertisement. If its own configuration revision number is higher than the request, it ignores the advertisement. If the configuration revision number is higher than its own, it sends an advertisement request.
ð VTP Subset Advertisement
Changes made to VLANs will increment the configuration revision number and issue a Summary Advertisement followed by one or more subset advertisements. The Subset Advertisement is what holds the VLAN information.It carries actual change in vlan information in domain in form of Vlan.dat file.
ð Advertisement request from Client
When a switch needs VTP information it sends an Advertisement Request. This request is sent if the switch has been reset, VTP domain name has changed, or it received a VTP Summary Advertisement with a higher configuration revision. Following the Summary Advertisement are Subset Advertisements.
ð VTP join message
VTP join message is send to solicitate what VLANs the downstream switch is using (local Deployment method). The purpose of VTP prunning feature is to cut the amount of unnecessary traffic that anyways would likely be dropped if ever reach the destination. VTP prunning feature do this by querying the device necessary VLANs.
It is enabled in Server and VLAN 1 can't be pruined.
Q: What is Synchronization Problem in VTP?
If any client switch is added on network with higher Cr value then the server will get updated from client. If this happens in real life then whole network goes down and is called synchronization problem in VTP.
To prevent such if we are adding any switch in network then we should ensure the CR number is 0. Question arises is how to make CR value 0. It's quite simple: change VTP Domain name.