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Every 802.1q trunk port has a native VLAN — but why does it exist? Simple: Untagged traffic needs somewhere to go. Not everything crossing a trunk is carrying a VLAN tag. Protocols like CDP (Cisco Discovery Protocol), LLDP (Link Layer Discovery Protocol), STP BPDUs (Spanning Tree Protocol), DTP (Dynamic Trunking Protocol), VTP (VLAN Trunking Protocol), and UDLD (Unidirectional Link Detection) send frames untagged. The switch needs to know what VLAN to assign them to. No tag? Must be native.
By default, Cisco’s native VLAN is VLAN 1 — but don’t mix up the native VLAN with the default VLAN (even though they’re both VLAN 1 by default). The default VLAN is hardcoded. The native VLAN is whatever you say it is — just make sure both sides of the trunk agree.
👉 Best practice: Set the native VLAN to an unused VLAN to avoid security risks — because no one wants untagged traffic wandering around where it doesn’t belong.
Check out this Cisco Press article, it also has a packet tracer lab at the bottom!
👉 Cisco Press
You Should Know:
Here are some practical commands and configurations related to VLANs and trunking on Cisco devices:
1. Configure Native VLAN on a Trunk Port:
interface GigabitEthernet0/1 switchport mode trunk switchport trunk native vlan 999
2. Verify Trunk Configuration:
show interfaces trunk
3. Change the Default Native VLAN:
interface GigabitEthernet0/1 switchport trunk native vlan 100
4. Check VLAN Assignment:
show vlan brief
- Disable DTP (Dynamic Trunking Protocol) on a Port:
interface GigabitEthernet0/1 switchport nonegotiate
6. Enable VLAN Trunking Protocol (VTP) Transparent Mode:
vtp mode transparent
- Verify STP (Spanning Tree Protocol) on a VLAN:
show spanning-tree vlan 1
8. Configure LLDP (Link Layer Discovery Protocol):
lldp run interface GigabitEthernet0/1 lldp transmit lldp receive
9. Check CDP (Cisco Discovery Protocol) Neighbors:
show cdp neighbors
10. Configure UDLD (Unidirectional Link Detection):
interface GigabitEthernet0/1 udld aggressive
What Undercode Say:
Understanding the role of the native VLAN in trunking is crucial for network security and efficiency. By configuring the native VLAN to an unused VLAN, you can prevent potential security risks associated with untagged traffic. Always ensure that both ends of a trunk link agree on the native VLAN configuration to avoid miscommunication. Additionally, regularly verify your VLAN and trunk configurations using commands like `show interfaces trunk` and `show vlan brief` to maintain a secure and efficient network environment.
For further reading and hands-on practice, refer to the Cisco Press mentioned earlier. It provides a detailed explanation and a packet tracer lab to reinforce your understanding.
References:
Reported By: Mohamed Abdelgadr – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
