Point	Trunk
Purpose	Multiple VLANs over one link
Standard	802.1Q
Frames	Tagged
Native VLAN	Untagged frames
Access port	One VLAN only
DTP	Cisco only
Typical failures	Native VLAN mismatch, trunk mismatch, allowed-VLAN mismatch

Remember: a trunk created without an allow statement carries all VLANs.

switchport nonegotiate disables DTP to prevent automatic trunk negotiation and improve security.

Configuration

! Basic trunk
interface GigabitEthernet0/1
 switchport mode trunk
!
! Trunk with native VLAN
interface GigabitEthernet0/1
 switchport mode trunk
 switchport trunk native vlan 99
!
! Trunk with allowed VLANs
interface GigabitEthernet0/1
 switchport mode trunk
 switchport trunk allowed vlan 10,20,30
!
! Remove DTP (best practice / CCNA)
interface GigabitEthernet0/1
 switchport mode trunk
 switchport nonegotiate
!
! Access port (contrast – classic trap)
interface GigabitEthernet0/2
 switchport mode access
 switchport access vlan 10

Verification (exam!) ⚠️

show interfaces trunk
show running-config interface g0/1