802.1Q Tunnels
Tunneling is a way to send 802.1Q-tagged frames across a foreign network
(such as a Service Provider’s network) and still preserve the original 802.1Q
tag (see Figure 2-3). The SP configures their end of the trunk link as a tunnel
port and assigns a VLAN to carry your traffic within their network. The SP
switch then adds a second 802.1Q tag to each frame that came in the tunnel
port. Other switches in the SP network see only this second tag, and do not
read the original tag. When the frame exits the SP network, the extra tag is
removed, leaving the original 802.1Q tag to be read by the receiving switch
in your network.