~/netref / VLANs & Trunking
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VLANs & Trunking

A VLAN is a logical broadcast domain. Trunks carry many VLANs between switches using 802.1Q tags.

// 802.1Q trunk

SW1
VLAN 10VLAN 20
802.1Q trunk
VLAN 10,20,30 · native 99
SW2
VLAN 10VLAN 20
One physical link carries many VLANs — each frame tagged with its VLAN ID (native VLAN travels untagged).
// VLAN ID ranges
0, 4095Reserved — never use
1Default VLAN (all ports)
2–1001Normal range
1002–1005Legacy Token Ring / FDDI
1006–4094Extended range

// trunking facts

  • 802.1Q inserts a 4-byte tag — 12-bit ID = 4094 usable VLANs
  • Native VLAN crosses a trunk untagged (default 1)
  • Access port = 1 VLAN · trunk = many VLANs
  • Inter-VLAN routing: router-on-a-stick or L3 SVI
  • VTP modes: server / client / transparent
  • Native VLAN must match on both trunk ends
vlan & trunk · cisco ios 
vlan 100
 name Engineering
!
interface Gi0/1
 switchport mode access
 switchport access vlan 100
 switchport voice vlan 150
!
interface Gi0/24
 switchport mode trunk
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 10,20-30
 switchport trunk native vlan 99
!
interface vlan100
 ip address 192.168.100.1 255.255.255.0
verify 
show vlan brief
show interfaces trunk
show interfaces switchport
show vtp status

// Q-in-Q (802.1ad) — VLAN stacking

DST/SRC MAC
S-Tag
provider
C-Tag
customer
Type / Payload
Two stacked 802.1Q tags: the outer S-Tag is added by the provider, the inner C-Tag is the customer original.

// facts

  • 802.1ad (Q-in-Q) stacks two 802.1Q VLAN tags
  • Outer S-Tag = provider VLAN · inner C-Tag = customer VLAN
  • Carries an entire customer VLAN range transparently across a provider
  • Adds 4 more bytes per tag — mind the MTU
  • Also called VLAN stacking or provider bridging