First of all let me show you a picture of a network:
Let’s start with a simple network topology:
Normally your switch will automatically learn MAC addresses and fill its MAC address table (CAM table) by looking at the source MAC address of incoming frames and flooding frames if it doesn’t know where to forward the frame.
When you want VLAN traffic between two switches then there is one problem we run into…take a look at the image below:
Trunks are required to carry VLAN traffic from one switch to another. In this lesson I will demonstrate how to configure a trunk between Cisco Catalyst switches. Let me show you the topology that we’ll use:
DTP is normally used on Cisco IOS switches to negotiate if the interface should become an access port or trunk.
By default DTP is enabled and the interfaces of your switches will be in “dynamic auto” or “dynamic desirable” mode. This means that whenever you receive a DTP packet that requests to form a trunk, your interface will be in trunk mode.
The IEEE 802.1Q trunking protocol describes something called the “native VLAN”. All traffic sent and received on an interface that is configured for 802.1Q won’t have a tag on its Ethernet frame. When you look at it in Wireshark, it will look the same just like any normal Ethernet frame.
Let’s say you have a network with 20 switches and 50 VLANs. Normally you would have to configure each switch separately and create those VLANs on each and every switch. That’s a time consuming task so there is something to help us called VTP (VLAN Trunking Protocol). VTP will let you create VLANs on one switch and all the other switches will synchronize themselves.
The main goal of VTP version 3 remains to synchronize VLANs but it has a number for extras. It’s been around for a while but until recent IOS versions it wasn’t supported on Cisco Catalyst Switches.
Here are some of the new additions to VTP version 3:
The protected port is a feature on Cisco Catalyst Switches that you can use to prevent interfaces from communicating with each other. Let me show you a picture to explain this:
Let’s get started with a nice topology picture: