This selects which vendor-defined encapsulation algorithm, the specific tagging or framing method, the switch applies when it wraps outgoing packets or strips headers from incoming packets on this VLAN encapsulation interface. Mismatched encapsulation types between two ends of a trunk link will cause frames to be silently dropped or misinterpreted rather than switched correctly. An admin connecting this switch to a neighboring device that expects 802.1Q tagging would need to set this to the enumeration value corresponding to that tagging scheme, not a mismatched proprietary one.