Virtual interfaces exist for configuration where there is no single physical attachment to a broadcast domain.
For switches with Layer 2 interfaces, VLANs define broadcast domains.
The VLAN interface is a Layer 3 interface for any member of the given VLAN.
For switches or routers with Layer 3 interfaces, broadcast domains are defined as bridge groups.
To route between bridge groups and other broadcast domains, a bridged virtual interface (BVI) is used as a Layer 3 interface.
In some instances, a physical Layer 3 interface can support traffic from multiple VLANs.
To provide Layer 3 interfaces for each VLAN on the physical connection, a subinterface is configured as the Layer 3 interface for the members of the VLAN.