Skip to main content

VLANs and trunking

Configuring VLANs and trunking on a Cisco device.

Terminology​

  • Standard VLAN: The default VLAN with an id of 1
  • Native VLAN: The VLAN used for untagged traffic (default is standard)
  • Data VLAN: Seperated VLAN to ensure secure data transfer
  • Management VLAN: A VLAN exclusively for network administration (SSH, telnet, SNMP)
  • Voice VLAN: A seperate VLAN for voice traffic
  • Tagged / Untagged: VLAN tags specify which VLAN a packet belongs to
  • Access ports: Ports tagging incoming packets to belong to a VLAN (allow a single VLAN)
  • Trunking ports: Ports that receive tagged packets (allow multiple VLANs)

Configuration​

! Create a vlan (can also be implicitly created on usage)
vlan 42
  name Production

Switches​

VLAN interface config​

interface vlan 42
  ip address 192.168.0.254 255.255.255.0
  no shutdown

Access ports​

interface fa0/1
  switchport mode access
  switchport access vlan 42

Trunking ports​

interface fa0/1
  switchport mode trunk
  switchport trunk native vlan 99
  switchport trunk allowed vlan 10,20,30

Other switchport modes​

config-if#
! Enable dynamic trunking
switchport mode dynamic auto

! Desire turning into a trunk
switchport mode dynamic desirable

! Disable dynamic trunking (no DTP frames)
switchport nonegotiate

Routers​

On a router trunking is done via subinterfaces.

! The base interface has to be enabled
interface gi0/0/0
  no shutdown

interface gi0/0/0.42
  description The subinterface for VLAN 42
  encapsulation dot1Q 42
  no shutdown

Typically one would only configure these on interfaces connected to a switch. Preserving VLAN tags between routers is unnecessary in most cases.

Voice VLANs​

vlan 10
  name Financing

vlan 100
  name Voice

interface fa0/18
  switchport mode access
  switchport access vlan 10

  ! Enables qos classification for IP phones
  mls qos trust cos
  switchport voice vlan 100

Evaluation​

#
show vlan brief

show vlan summary