Red Hat Enterprise Linux machines are often used as routers and enable an advanced VLAN configuration on their network interfaces. You need to set
switchport mode when the Ethernet interface is connected to a switch and there are VLANs running over the physical interface. A Red Hat Enterprise Linux server or workstation is usually connected to only one VLAN, which makes
switchport mode access suitable, and the default setting.
In certain scenarios, multiple tagged VLANs use the same physical link, that is Ethernet between the switch and Red Hat Enterprise Linux machine, which requires
switchport mode trunk to be configured on both ends.
For example, when a Red Hat Enterprise Linux machine is used as a router, the machine needs to forward tagged packets from the various VLANs behind the router to the switch over the same physical Ethernet, and maintain separation between those VLANs.