Works with layer 3 switches. Certain ports on a switch might be associated with different VLANs.
Switches ’tag’ frames as they pass through VLAN trunking. Tags get removed as frames pass from trunking to end stations.
The 802.1q protocol is used for implementing VLANs.
Notes #
- Untagged frames belong to the native VLAN.
- Cisco IP phones have their own VLAN ID to separate it from regular traffic.
- Dynamic Trunk Protocol allows a port to negotiate it’s mode (access or trunk).
Modes #
By default, switches use dynamic auto.
| Dynamic Auto | Dynamic Desirable | Trunk | Access | |
|---|---|---|---|---|
| Dynamic Auto | Access | Trunk | Trunk | Access |
| Dynamic Desirable | Trunk | Trunk | Trunk | Access |
| Trunk | Trunk | Trunk | Trunk | — |
| Access | Access | Access | — | Access |
Setup #
Switch:
interface range fa0/5 - 10
switchport mode access
# SET A PORT TO TRUNK MODE AND SET NATIVE VLAN
interface fa0/1
switchport mode trunk
switchport trunk native vlan 99
# CREATE A VLAN
vlan 20
name Sales
# ASSIGN A RANGE OF PORTS TO A VLAN
interface range fa0/13 - 24
switchport mode access
switchport access vlan 20
# VOICE VLAN
switchport voice vlan 99
show vlan brief
delete vlan.dat
To fix native VLAN mismatch, the native VLAN must be configured the same on each interface connected to a hub so that generated traffic is only redirected to one VLAN: switchport trunk native vlan X.
Inter-VLAN Routing #
Router:
interface fa0/0.10
encapsulation dot1q 10
ip address 192.168.10.1 255.255.255.0
interface fa0/0.20
encapsulation dot1q 20
ip address 192.168.20.254 255.255.255.0
Layer 3 switches can route packets between VLANs.