VLANS & VTP In a Nutshell

Virtual LANS provide a way to split a single physical LAN into multiple broadcast domains. This reduces the number of broadcast frames that each device must process and that each switch must forward. Thus reducing congestion on the network. VLANS are also helpful in providing security by isolating devices that handle sensitive data from the rest of the network.


You can have a single switch with five interfaces (and the devices attached to them) assigned to VLAN 1 and another five interfaces assigned to VLAN 10. You can also assign IP phones to their own VLAN thus separating voice traffic from regular data.

If you would like the multiple VLANS throughout your entire network you would need to configure those VLANs on all switches in the network. In large networks that can be very  laborious since some large networks have hundreds of switches. The solution to this problem is the VLAN Trunking Protocol - VTP.

VTP allows you to create a VLAN on one switch and have it automatically replicated to all other switches (or a subset of switches). Once two switches are in the same VTP domain and have the same password (if this option is enabled) they will be able to exchange VLAN information.

Each time a switch learns of a change to the VLAN database it increments a "revision number" upward by 1 and all other switches synchronize.

NB. It is important that new switches attached to an existing network have a lower revision number than the currently network-wide revision number, otherwise the network switches will synchronize to use the new switches VLAN database and erase their own database.

VTP configuration is relatively simple. Take a look at the example below:

"vtp mode server" enables the device to initiate vtp updates and is the default. The two other modes are client and transparent. Clients cannot create or delete vlans but they receive and apply VTP updates.

Transparent mode switches do not initiate updates nor do they apply updates that they receive, but they do forward updates that they receive to other devices. When transparent mode is configured on a switch it effectively disables vtp on that switch.

So there you have it, VLANS & VTP in a nutshell, I hope this was informative.


All the best,

Roger.


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