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Configuring a trunk on a switch s optical port

ODN and passive infrastructure knowledge brief Published 31 Aug 2023

Prepared for preliminary research by the GESTIONVAL ODN knowledge desk. Confirm network topology, split ratio, box or cabinet capacity, joint closure rating and loss budget against current standards and product documentation.

To configure a Cisco switch optical port as a trunk, set the interface to trunk mode, specify 802.1Q encapsulation, and define allowed VLANs.Step-by-Step Configuration

Access the Switch CLI
- Connect via console, SSH, or Telnet.
- Enter privileged EXEC mode:
  enable
- Enter global configuration mode:
  configure terminal
Select the Optical Interface
- Identify the optical port (e.g., GigabitEthernet1/1 or TenGigabitEthernet1/1):
  interface GigabitEthernet1/1
Specify Trunk Encapsulation
- Set the encapsulation to IEEE 802.1Q (industry standard):
  switchport trunk encapsulation dot1q
- Note: Some older switches may require enabling dot1q explicitly before trunking.
Set the Port to Trunk Mode
- Configure the interface as a permanent trunk:
  switchport mode trunk
- Avoid dynamic modes (dynamic auto or dynamic desirable) for security and predictability.
Configure Allowed VLANs
- Specify which VLANs can traverse the trunk:
  switchport trunk allowed vlan 10,20,30
- This ensures only required VLAN traffic passes through the trunk.
Set the Native VLAN
- For security, assign a native VLAN that is unused:
  switchport trunk native vlan 1001
- Frames without a VLAN tag will be assigned to this VLAN.
Verify the Configuration
- Exit interface configuration mode:
  end
- Check trunk status:
  show interfaces trunkshow running-config
Save the Configuration
write memory or
copy running-config startup-config

Best Practices

Disable DTP if connecting to non-Cisco devices to prevent unwanted trunk negotiation.
Ensure VLAN consistency on both ends of the trunk to avoid spanning-tree loops.
Use 802.1Q instead of ISL for interoperability with modern devices.
Verify loop-free topology before enabling trunking on production networks. By following these steps, the optical port will carry traffic for multiple VLANs, enabling efficient inter-switch communication and VLAN extension across the network .

Configuring a trunk on a switch s optical port - GESTIONVAL ODN INFRASTRUCTURE S.L. [PDF]

Configuring a trunk on a switch s optical port

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Technical note

This reference is intended for preliminary ODN and passive infrastructure research. Topology, split ratio, box or cabinet capacity, closure rating, cable type, test limits and applicable standards must be verified for the specific project.

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