VLAN Trunking Protocol (VTP) is a replicating protocol for multi-switch VLAN trunks, and is a Cisco proprietary protocol. The connection between two switches is called a trunk and the traffic over the trunk follows the process of VLAN tagging and adds a VLAN identifier (VLAN ID) to the packets. It helps keep the data of different VLANs separate. Cisco supports two trunking protocols to enable interoperability of VLANs, 802.1Q – an IEEE defined protocol, and ISL (Inter-Switch Link) – a Cisco proprietary, and we can tag 4094 VLANs using these two protocols. But, these still do not reduce the overhead of configuring VLANs individually on all the switches.
This is where VTP comes to the rescue. It helps to maintain the consistency of the VLANs throughout the network. By using VTP we can create VLANs on all the switches in a network, at the same time, from a centrally managed location. In simple terms, if VTP is enabled, the creation of a new VLAN on one switch results in the creation of the same VLAN on all the switches in the same VTP management domain. This saves the network engineer from committing errors while configuring VLANs on every switch and even saves the valuable time. A switch is not a part of any VTP management domain by default, and it can be a part of only one such domain at a time. Once VTP has been configured on a switch, it operates in the server mode by default, that is, it is used to add, delete and change VLANs in the VTP management domain. We can also configure a switch to VTP client mode – to receive the changes done by the server mode, or also to transparent mode – to pass on the information to other switches. On new switches, a VTP can be disabled as well. The VTP updates or advertisements are sent every 5 minutes over the domain, or whenever there is a change in the configurations. We can also implement VTP Pruning to stop a switch from passing traffic for a VLAN that is not even configured on the other switch. To conclude, it can be said that the VTP has simplified the administration of a network by streamlining the process of managing VLANs over the trunk and reducing the possibility of occurrence of errors. But, it should be used with extreme caution as one change on a VTP server will reflect in the entire management domain of that VTP.
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