===== VLANs =====
  * On a router, if a physical interface is configured with many VLANs, then sub-interfaces have to be used. Additionally, the switchport port must be a trunk. An access port will not work, I tested this.
    * Router(config)#** interface g0/0.100**
    * Router(config-subif)#** encapsulation dot1q 100**
    * Router(config-subif)#** ip address 100.1.1.1 255.255.255.0**
    * ---
    * Switch(config)#** interface g1/0/1**
    * Switch(config-if)#** switchport trunk encapsulation dot1q**
    * Switch(config-if)#** switchport mode trunk**
    * Switch(config-if)#** switchport trunk allowed vlan 100**

  * A single vlan doesn't need to belong to a dot1q trunk to work.
    * Router(config)#** interface g0/1**
    * Router(config-if)#** ip address 200.1.1.1 255.255.255.0**
    * ---
    * Switch(config)#** interface g1/0/2**
    * Switch(config-if)#** switchport mode access**
    * Switch(config-if)#** switchport access vlan 200**