Advanced VLAN Configuration

i need a superhighway that destroys all the poor neighborhoods but instead it's a network

ok so since VLANs need to be explicitly (logically) connected to each other even though they might be on the same physical interface

we need to be able to connect switches with VLANs together for redundant links since we're a big boy network. these connections are called trunks.

802.1Q

but how do we route packets? this is where 802.1Q comes in, and it basically adds a VID (VLAN ID) tag to the Ethernet packet frame. It's in between the Source Address and EtherType fields, and the EtherType is set to 802.1Q.

VLAN Port Tagging

if a switch port will only ever be in a single VLAN, it's marked as untagged. its also referred to as an access/host port.

• if a frame is address to a port in the same VLAN on the same switch, tagging isn't required per 802.1Q
• if a frame needs to be forwarded over a trunk link, then it needs a port tag under 802.1Q

Default vs. Native VLANs

Routing

Router on a stick topology. I am NOT joking. This is NOT a drill. The router is deployed with single interface that's connected to a trunk port on the switch. This trunk port carries all the VLAN-to-VLAN traffic that needs to be routed. Now the router's physical interface has multiple subinterfaces, each configured with it's own VID and IP address and acting as the default gateway for it's VLAN.

Configure a subinterface for VLAN16 on G0, using the last available address as the default gateway:

• interface G0.16
• encapsulation dot1Q 16
• ip address 198.51.100.30 255.255.255.240

DHCP Configuration

There are special cases in which the DHCP services of the network are on a different VLAN than the client devices. In this case, a special link between the DHCP VLAN and the client VLAN must be created, called a DHCP relay. Relays are set using the ip helper-address [DHCP server ip address] command using Cisco IOS.