1. Introduction to Networking
Trunking

2. Trunking
If we have multiple VLANs spanning multiple switches we will have a problem due to our “one VLAN per port” restriction
This would mean that we would need a dedicated port for each VLAN to connect the switches. This would take up too many ports in both of our switches
To get around this, we use trunk ports

3. Trunk Ports
A trunk port is a switch port that connects one switch to another
Trunk ports can be members of multiple VLANs, which allow them to pass messages for any VLAN it is a member of from one switch to another
When created a trunk port is naturally connected to every VLAN on the switch, but we can limit that if we need to

4. VLAN Tags
To ensure that messages sent from one switch to another stay within their own broadcast domain, the switch will append a VLAN tag to the frame
These IDs are only attached to frames going over trunk ports to let the connected switch know what VLAN the frame originated from
After the switch receives the frame, it will remove the VLAN tag and treat the frame as having come from the trunk port on the appropriate VLAN

5. VTP
There are ways for us to automate some management of our multi-switch VLAN using VTP (VLAN Trunking Protocol)
VTP allows us to propagate configuration changes throughout our network automatically
With VTP each switch will be assigned a configuration mode; either server, client or transparent

6. VTP Modes
Server - Any changes done to this switch will propagate to the other switches in the network
Client - Any changes done to a connected server switch will be applied to this switch. Also, any time a server changes this switch’s configuration it will pass that configuration along the network to other switches. It’s important to note that changes can not be made to this switch directly though
Transparent - You can configure this switch locally and it will ignore any incoming server switch changes, however it will pass the changes along the network for other client switches
By default most switches start in server mode. If you don’t plan to use VTP you should switch the mode to transparent

7. 802.1Q
This is a widely implemented trunking standard that supports a wide range of switches and manufacturers
Frames from the default VLAN are not tagged, but all other VLANs are tagged. This means that all switches on the network should have the same VLAN as the native VLAN, or there will be network disconnects

8. Trunk Connection Negotiations
Out of the box, most switches will allow for trunking automatically
The protocol for this is DTP (Dynamic Trunking Protocol)
There are three modes for ports to negotiate trunk status -
Access - This port will ignore DTP and always be an access port
Trunk -This port will ignore DTP and always be a trunk port
Dynamic - DTP will choose if this port is access or trunk
A dynamic port has two options -
Auto - If both sides are auto, then the trunk will not become active
Desirable - if at least one side is desirable, the trunk connection will activate

9. Trunking Commands Show interface trunk - shows detail on trunk ports
Switchport mode trunk switches a port from access to trunk
Switchport mode access - switches a port from trunk to access
Switchport trunk allowed
Add [VLAN] - adds the selected VLAN
Remove [VLAN] - removes the selected VLAN
Switchport trunk native VLAN [VLAN] - Sets the native VLAN to VLAN