Introduction to Networking Trunking

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

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

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

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

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

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

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

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