VLANs create separate broadcast domains and isolates traffic to it's own VLAN. Switches will not forward traffic out ports that aren't in their VLAN. Basically, traffic from VLAN 10 cannot reach VLAN 20. Makes grouping traffic pretty easy. For example you may group ACCOUNTING and IT machines into different VLANs.

Trunking is the act of carrying data from multiple VLANs over a medium. So a trunk port is a port capable of carrying traffic from specified/any VLANs. This is useful if in a multi-switch environment or inter-vlan routing / RoAS.

We'll configure this topology as shown.

We'll configure ports 4 and 8 on the switch into their VLANs, also 4 and 8 to keep it simple.

** Configure VLANs
Switch>en
Switch#conf t
Switch(config)#interface fastEthernet 0/8
Switch(config-if)#switchport access vlan 8
% Access VLAN does not exist. Creating vlan 8

Switch(config-if)#int fa0/4
Switch(config-if)#switchport access vlan 4
% Access VLAN does not exist. Creating vlan 4

Switch# show vlan brief
 ** WILL CONFIRM VLANs

At this point the PCs are in their own VLAN. In order for them to communicate we must set up a trunk port to the router to allow for inter-VLAN routing.

Set switchport Fa0/1 to TRUNK mode to carry all our VLAN traffic. Remember that the switchport mode by default is dynamic desirable which will enable trunking by default if connected to another switch, AKA BAD.

** Setup trunk port to Router
Switch(config-if)#int fa0/1
Switch(config-if)#switchport mode trunk 
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to up

Now to configure trunking on the router. We do so with subinterfaces to put an interface in each VLAN we want to route traffic on.

 ** Set trunk on Router with Sub-Interfaces
Router(config)#interface fastEthernet 0/0
Router(config-if)#no shutdown 
%LINK-5-CHANGED: Interface FastEthernet0/0, changed state to up
%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up

Configuring our subinterfaces by creating them, setting the encapsulation (dot1q = trunking), and finally an IP address.

Router(config)#interface fastEthernet 0/0.4
Router(config-subif)#
%LINK-5-CHANGED: Interface FastEthernet0/0.4, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0.4, changed state to up

Router(config-subif)#encapsulation dot1Q 4
Router(config-subif)#ip address 10.0.4.1 255.255.255.0



Router(config-subif)#int fa0/0.8
%LINK-5-CHANGED: Interface FastEthernet0/0.8, changed state to up

%LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0.8, changed state to up


Router(config-subif)#encapsulation dot1Q 8
Router(config-subif)#ip address 10.0.8.1 255.255.255.0

 ** Ensure our networks are connected and in the table
Router#show ip route
 ** CODES OMITTED

     10.0.0.0/24 is subnetted, 2 subnets
C       10.0.4.0 is directly connected, FastEthernet0/0.4
C       10.0.8.0 is directly connected, FastEthernet0/0.8