The Lazy Networker

In VLAN configurations, trunking allows multiple VLANs to be transmitted over a single physical link between switches. Trunking can be set up using static trunking or dynamic trunking.

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1. Static Trunking

Static trunking means manually configuring a switch port as a trunk. This ensures that the port remains in trunk mode, allowing multiple VLANs to pass through it.

Advantages of Static Trunking

✅ More secure (prevents unauthorized devices from negotiating trunks).
✅ No risk of dynamic negotiation failures.
✅ Preferred in enterprise networks for stability.

Configuration of Static Trunking (Cisco Switch Example)

bashCopyEditinterface GigabitEthernet0/1
  switchport mode trunk
  switchport trunk allowed vlan 10,20,30
  switchport trunk native vlan 99
  exit

👉 This command sets GigabitEthernet0/1 as a static trunk, allowing VLANs 10, 20, and 30, and setting VLAN 99 as the native VLAN.

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2. Dynamic Trunking (DTP – Dynamic Trunking Protocol)

Dynamic trunking allows ports to negotiate whether to become a trunk or remain an access port. Cisco switches use DTP (Dynamic Trunking Protocol) to automate trunk formation.

DTP Modes

Mode	Behavior
Access	Forces the port into access mode (no trunking).
Trunk	Forces the port into trunk mode (like static trunking).
Dynamic Auto	Waits for the other side to initiate trunking but does not actively negotiate.
Dynamic Desirable	Actively tries to negotiate trunking with the other device.

Configuration of Dynamic Trunking (Cisco Example)

interface GigabitEthernet0/2
  switchport mode dynamic desirable
  exit

👉 The desirable mode will actively try to establish a trunk if the other side supports it.

interface GigabitEthernet0/3
  switchport mode dynamic auto
  exit

👉 The auto mode waits for the other side to initiate trunking. If both sides are set to auto, the trunk will not form.

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3. Key Differences: Static vs. Dynamic Trunking

Feature	Static Trunking	Dynamic Trunking (DTP)
Configuration	Manually set to trunk mode	Uses DTP to negotiate trunking
Security	More secure	Less secure (vulnerable to VLAN hopping attacks)
Stability	Always trunk	May fail to negotiate a trunk
Best Use Cases	Enterprise networks, high-security environments	Simple setups or mixed vendor environments

Best Practice Recommendation

🔹 Disable DTP on all trunk ports and configure static trunking for security.
🔹 Use switchport nonegotiate to prevent DTP from running:

interface GigabitEthernet0/4
  switchport mode trunk
  switchport nonegotiate
  exit