VLANs and Trunking Explained

VLANs and Trunking

What does Virtual Local Area Network (VLAN) mean?

Virtual LAN is a Layer 2 technique which allows you to divide your physical network into logical segments.The most  beneficial elements of a VLAN is that it removes latency in the network, which saves network resources and increases network efficiency. In addition, VLANs are created to provide segmentation and assist in issues like security, network management and scalability. Furthermore, traffic patterns can easily be controlled by using VLANs. 

Let’s see how to configure a simple virtual LAN .

Configuration

Switch 1		Ethernet Ports
Vlan 10 Interface IP	192.168.10.1	1-10
Vlan 10 Desktop IP	192.168.10.100
Vlan 20 Interface IP	192.168.20.1	11-18
Vlan 20 Desktop IP	192.168.20.100
Vlan 20 Interface IP	192.168.30.1	19-23
Vlan 10 Desktop IP	192.168.30.100

VLAN Configuration

Step 1
Create new VLANs 10, 20, 30 in Switch and assign an IP address to switched virtual interface or an SVI using the following sequence of configuration commands
To Create VLAN

Switch1(config)#vlan 10
Switch1(config-vlan)#name HR

Switch1(config-vlan)#vlan 20
Switch1(config-vlan)#name Sales

Switch1(config-vlan)#vlan 30
Switch1(config-vlan)#name IT

Assigning IP address to VLAN interface
Switch1(config)#interface vlan 10
Switch1(config-if)#ip address 192.168.10.1 255.255.255.0
Switch1(config-if)#no shutdown

Switch1(config)#interface vlan 20
Switch1(config-if)#ip address 192.168.20.1 255.255.255.0
Switch1(config-if)#no shutdown

Switch1(config)#interface vlan 30
Switch1(config-if)#ip address 192.168.30.1 255.255.255.0
Switch1(config-if)#no shutdown

Next step is to assign interfaces for VLANs 10,20,30 respectively 

Switch1(config)#interface range fastEthernet 0/1-10
Switch1(config-if-range)#switchport mode access
Switch1(config-if-range)#switchport access vlan 10
Switch1(config-if-range)#exit

Switch1(config)#interface range fastEthernet 0/11-18
Switch1(config-if-range)#switchport mode access
Switch1(config-if-range)#switchport access vlan 20
Switch1(config-if-range)#exit

Switch1(config)#interface range fastEthernet 0/19-23
Switch1(config-if-range)#switchport mode access
Switch1(config-if-range)#switchport access vlan 30
Switch1(config-if-range)#exit

Vlan is configured and assigned interfaces for vlan.If you want to communicate between  virtual LANs you need a enable ip routing in Layer 3 device.
To enable ip routing 

Switch1(config)#ip routing

Test this Scenario

Do a ping test from vlan 10 to other Vlans or Vise Versa.

Consider your office is extend and you have another Floor.
 Now you need to extend the VLAN to the second floor. Here you need to configure another layer 3 switch (switch 2) with the same configuration of switch 1 and need to connect cables from VLAN 10 ,VLAN 20 and VLAN 30 to the respective ports of switch 2.

Here you are wasting 3 port in switch 1 and 3 ports in switch 2,so total 6 ports. Also you are laying 3 cables. If you increase the VLAN then you waste more ports and cables. To over come this ,the better way is to setup a trunk port as described in the diagram.

A trunk is a point-to-point link between two network devices that carries more than one VLAN over a single link and the process known as trunking. With VLAN trunking, you can extend your configured VLAN across the entire network.

In this demonstration am using port 24 as trunk port. 

Configure the Switch 2 same as we done in switch 1 (VLAN Configuration-Step 1)

Switch 2		Ethernet Ports
Vlan 10 Interface IP	192.168.10.2	1-10
Vlan 10 Desktop IP	192.168.10.200
Vlan 20 Interface IP	192.168.20.2	11-18
Vlan 20 Desktop IP	192.168.20.200
Vlan 20 Interface IP	192.168.30.2	19-23
Vlan 10 Desktop IP	192.168.30.200
Trunk Port		24

VLAN Trunking

Step 1
Switch 1
Switch1(config)#interface fastEthernet 0/24
Switch1(config-if)#switchport mode trunk
Switch1(config-if)#switchport trunk allowed vlan 1-99

Switch 2
Switch2(config)#interface fastEthernet 0/24
Switch2(config-if)#switchport mode trunk
Switch2(config-if)#switchport trunk allowed vlan 1-99
Note : When you try to configure trunk port you might see following error
"An interface whose trunk encapsulation is Auto can not be configured to trunk mode" .To fix this you need set encapsulation to 802.1q using the following command in interface fastEthernet 0/24
Switch1(config-if)#switchport trunk encapsulation dot1q
This should fix your issue.

Test this Scenario

Do a ping test from second floor vlan 10 to other Vlans in First Floor or Vise Versa.