Layer 2 Protocol Tunneling (GBPT)
If a Service Provider separates sections of your network, you can use Layer
2 protocol tunneling to tunnel CDP, Spanning Tree Protocol (STP), and
VLAN Trunking Protocol (VTP) frames across the SP’s cloud. This is called
Generic Bridge PDU Tunneling (GBPT). Frames from the above control
protocols are encapsulated as they enter the SP’s network on a tunnel port,
and de-encapsulated when they exit that network.