Without L2PT, the handling of the PDUs will create different spanning tree domains (different
spanning tree roots) for the customer switches. To provide a single spanning tree domain for the
customer switches, a generic scheme to tunnel BPDUs was created for control protocol PDUs.
This process is referred to as Generic Bridge PDU Tunneling (GBPT).
GBPT provides a scalable approach to PDU tunneling by software encapsulating the PDUs in the
ingress edge switches and then multicasting them in hardware. All switches inside the service
provider network treat these encapsulated frames as data packets and forward them to the other
end. The egress edge switch listens for these special encapsulated frames and decapsulates them;
they are then forwarded out of the tunnel.
The encapsulation involves the rewriting of the destination media access control (MAC) address in
the PDU. An ingress edge switch rewrites the destination MAC address of the PDUs received on a
Layer 2 tunnel port with the desired multicast address.
Layer 2 Protocol Tunneling: Enable or disable the Layer 2 Protocol Tunneling fuction globally.
Destination MAC Address: Specify a MAC address for GBPT. User- defined. Default is
01:00:0C:CD:CD:D0.
Class of Service: There are eight priority levels (0~7) that you can choose to classify data
packets. Specify the preferred priority bit value as L2PT class of service (cos). The default value is
“5”.
CDP, LLDP, STP, VTP, LACP, PAgP, UDLD: Configure the Layer 2 port as a Layer 2 protocol
tunnel port by clicking on the corresponding PDUs’ checkbox for the preferred port. To configure
all ports with the same PDU at a time, just click on the checkbox of the desired PDU in All port row.
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