Understanding VLAN Trunking Protocol (VTP) in Cisco Switch

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VLAN Trunking Protocol (VTP) is used to synchronize VLAN creation between the switches in the network. Imagine you have dozens, if not hundreds of switches in your network. You have to log into each switch and manually create VLANs. This does not sound practical, does it? Using VTP protocol, you can manually create VLANs in one switch and these VLANs will be replicated to all other switches in the particular VTP domain. VTP protocol simplifies creation, deletion and addition of VLANs among group of switches that are connected via TRUNK links with each other. VTP is Cisco proprietary and only supported on Cisco Switches.

Understanding VLAN Trunking Protocol (VTP) in Cisco Switch

Before you start configuring VTP, you need to understand what VTP is and how it can be helpful and disastrous at the same time in certain conditions. The diagram below shows a simple topology, where SwitchA is connected to SwitchB and SwitchC, SwitchC is connected to SwitchD via TRUNK links.

Understanding VLAN Trunking Protocol (VTP) in Cisco Switch

VTP uses TRUNK links to advertise VLAN information to other switches. So, if you don’t have TRUNK link between switches VTP is not going to work. VTP advertisements contains VLAN information, domain name, revision number, etc.

VTP Domains

VTP domain is a management domain with common VLAN requirement. By default, the domain name is NULL. You have to configure same VTP domain names between switches where you want the VLANs to be replicated. So, in above diagram all switches needs to be on same VTP domain for running VTP protocol. If you configure VTP domain say mustbegeek.com in SwitchA, the domain name will automatically replicate to SwitchB, SwitchC and SwitchD. This is default behavior. If you want different VTP domain name you can log into say SwitchB and change it manually. Remember, VTP only works via TRUNK links.

VTP Modes

There are three different VTP modes. These modes defines what the switch is able to do in terms of VLAN replication.

  1. Server Mode: – In server mode, you can create VLANs in the switch and these VLANs will be replicated to other switches that have VTP mode server or client in the same domain. By default, VTP in all switches are operating in Server mode. Switches in server mode sends VTP advertisements to all other switches. When you want to create VLANs, you log into Switch that has VTP Mode – Server and create VLANs.
  2. Client Mode: – In client mode, you can not create or delete VLANs in the switch. Switches that are in client mode receives VTP advertisements from switches that are in server mode and rely it to other switches as well. So, whatever VLANs are created in switches in server mode are automatically created in switches that are in client mode.
  3. Transparent Mode: – Switches that are in transparent mode do not re-create VLANs that are already configured in switches operating in VTP server mode. In VTP version 2, switches in transparent mode do relay VTP advertisements received from switches that are in server mode to other switches. You can manually create VLAN in switches that is in VTP transparent mode.

In above diagram, SwitchA can be in server mode, SwitchB, SwitchC and SwitchD can be in client mode. In this case, when you create VLANs in SwitchA, those VLANs will be replicated and created in SwitchB and SwitchC as well.

There are three VTP versions.

  1. v1: – Cisco switches use VTP v1 by default. VTP v1 supports VLAN numbers from 1 to 1005
  2. v2: – v2 also supports VLAN numbers from 1 to 1005. It also supports token ring. Transparent mode now relays VTP advertisements to other switches.
  3. v3: – v3 supports all VLANs from 1 to 4094. Supports VTP authentication, Multiple Spanning Tree (MST) configuration and private VLANs. Gives option to disable VTP globally or per link. It introduces concept of primary and secondary VTP Server.

VTP Synchronization

In order to synchronize VTP information, VTP advertisements uses VTP revision number to advertise latest updates to other switches. Switches that have higher revision number sends VTP advertisements to switches that have lower revision number. So always make sure the revision number is lower when you are adding new or old switch in the network. You can manually make revision number to 0. To do so, change the switch’s VTP mode to transparent and then change back to server or client.

Once you’ve understood what VTP is, you can easily configure VTP in Cisco Switches.

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Bipin is a freelance Network and System Engineer with expertise on Cisco, Juniper, Microsoft, VMware, and other technologies. You can hire him on UpWork. Bipin enjoys writing articles and tutorials related to Network technologies. Some of his certifications are, MCSE:Messaging, JNCIP-SEC, JNCIS-ENT, and others.

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