25.1.2 Prerequisites for using VLAN tunneling
The use of the VLAN tunneling functionality requires a network configured to meet all of the following conditions:
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A port VLAN is used.
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On the VLAN implementing the VLAN tunneling functionality, the tunneling port is on the access line, and the trunk port is on the backbone line.
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Because VLAN tags are stacked on the backbone line within the VLAN-tunneled network, frames that are 4 bytes larger than usual need to be handled.
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Access ports and tunneling ports cannot both exist within a switch. When at least one tunneling port is set, ports set as access ports also run as tunneling ports.