How to Configure a Trunk Port on a Cisco Switch
To configure a trunk: set the port to trunk mode, then optionally restrict the allowed VLANs and set the native VLAN. A trunk carries multiple VLANs between switches over one link.
Trunks link switches so VLANs span the whole network. See the trunk lab for hands-on practice.
Step 1: Set the port to trunk mode
interface gi0/1switchport mode trunk | Forces the port to trunk (carries all VLANs by default) |
Step 2: Restrict the allowed VLANs (recommended)
switchport trunk allowed vlan 10,20,99 | Only these VLANs cross — better security and less broadcast noise |
Step 3: Set a dedicated native VLAN
switchport trunk native vlan 99switchport nonegotiate | Untagged VLAN set off VLAN 1; DTP disabled to prevent spoofing |
Verification
show interfaces trunk confirms the port is trunking with the right native VLAN and allowed list. Same-VLAN devices on different switches should now ping across the trunk.
Frequently asked questions
How do I configure a trunk port on a Cisco switch?
In interface config: switchport mode trunk. Then optionally restrict VLANs with switchport trunk allowed vlan and set the native VLAN.
What is the native VLAN on a trunk?
The one VLAN whose traffic crosses untagged. Set it to a dedicated unused VLAN (not VLAN 1) on both ends to prevent VLAN-hopping.
Why use switchport nonegotiate?
It disables DTP so the port won't auto-negotiate trunking — closing a switch-spoofing attack vector.
How do I verify a trunk is working?
show interfaces trunk lists trunking ports, their native VLAN and allowed/forwarding VLANs.
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