802.1Q trunking carries multiple VLANs over a single link using tags; the native VLAN carries untagged frames.

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

Remember: creating a trunk without an “allow” statement means all VLANs are allowed.

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

Disable 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