Server connected to switch via trunk port

A server can be connected to a switch trunk port by configuring the switch port as a trunk and setting up VLAN subinterfaces on the server to handle multiple VLANs.

A trunk port on a switch carries traffic for multiple VLANs using IEEE 802.1Q tagging. To configure a switch port as a trunk:

• Set the port mode to trunk: switchport mode trunk
• Specify allowed VLANs: switchport trunk allowed vlan 10,20,30
• Configure the native VLAN for untagged traffic: switchport trunk native vlan 1
• If using EtherChannel, ensure the physical interfaces and the port-channel configuration match, and use channel-group xx mode on for static or mode active for LACP negotiation .

Trunk ports are typically used to connect network infrastructure devices, but servers can also act as trunk hosts if they are configured to handle multiple VLANs .

On a Linux server, VLAN trunking is implemented by creating tagged VLAN subinterfaces on the physical NIC connected to the trunk port:

• The physical interface (e.g., eth0) does not need an IP address; it handles untagged traffic for the native VLAN.
• Create subinterfaces for each VLAN: eth0.10, eth0.20, eth0.30, each with its own IP address and VLAN ID.
• Example configuration snippet using networkd:

ethernets: eth0: dhcp4: falsevlans: eth0.10: id: 10 link: eth0 addresses: [10.10.0.1/24] eth0.20: id: 20 link: eth0 addresses: [10.20.0.1/24] eth0.30: id: 30 link: eth0 addresses: [10.30.0.1/24]

This allows the server to send and receive traffic for multiple VLANs over a single physical link .

• Suspended ports: Often caused by misalignment between physical interfaces and port-channel configuration. Ensure the switch and server configurations match, especially for EtherChannel or LACP settings .
• Native VLAN mismatch: The native VLAN must be the same on both ends of the trunk to avoid spanning-tree loops .
• Trunk negotiation: If the server does not support DTP, use static trunking (switchport nonegotiate) on the switch .
• Load sharing: For multiple trunk links, configure STP port priorities or path costs to control which trunk carries traffic for specific VLANs .

• Trunk ports allow a single physical link to carry multiple VLANs.
• Servers must be configured with VLAN subinterfaces to handle tagged traffic.
• Ensure VLAN IDs, native VLAN, and EtherChannel settings are consistent between the switch and server.
• Proper configuration prevents suspended ports, spanning-tree loops, and VLAN misrouting.