L2PT is used to transparently transport protocol data units (PDUs) of Layer 2 protocols such as STP, CDP, DTP, VTP, PAGP, and UDLD. This allows running these protocols between customer CPEs without involving backbone infrastructure.
The 7210 SAS routers allow transparent tunneling of PDUs across the VPLS core. However, in some network designs, the VPLS PE is connected to CPEs through a legacy Layer 2 network instead of having direct connections. In such environments termination of tunnels through such infrastructure is required.
L2PT tunnels protocol PDUs by overwriting MAC destination addresses at the ingress of the tunnel to a proprietary MAC address such as 01-00-0c-cd-cd-d0. At the egress of the tunnel, this MAC address is then overwritten back to MAC address of the respective Layer 2 protocol.
The 7210 SAS nodes support L2PT termination for STP BPDUs. More specifically:
At ingress of every SAP which is configured as L2PT termination, all PDUs with a MAC destination address, 01-00-0c-cd-cd-d0 are intercepted and their MAC destination address are overwritten to MAC destination address used for the corresponding protocol. The type of protocol can be derived from LLC and SNAP encapsulation.
In egress direction, PDUs of the corresponding protocol received on all VPLS ports are intercepted and L2PT encapsulation is performed for SAPs configured as L2PT termination points. Because of the implementation reasons, PDU interception and redirection to CPM can be performed only at ingress. Therefore, to comply with the preceding requirement, as soon as at least 1 port of a specific VPLS service is configured as L2PT termination port, redirection of PDUs to CPM is set on all other ports (SAPs) of the VPLS service.
L2PT termination can be enabled only if STP is disabled in a context of the specific VPLS service.