L2PT is used to transparently transport protocol data units (PDUs) of Layer 2 protocols such as STP, CDP, VTP, PAGP, and UDLD. This allows running these protocols between customer CPEs without involving backbone infrastructure.
The 7450 ESS, 7750 SR, and 7950 XRS 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, and it does not have direct connections. In such environments, termination of tunnels through such infrastructure is required.
L2PT tunnels 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 the MAC address of the respective Layer 2 protocol.
The 7450 ESS, 7750 SR, and 7950 XRS routers support L2PT termination for STP BPDUs. More specifically:
At ingress of every SAP/spoke-SDP that is configured as L2PT termination, all PDUs with a MAC destination address of 01-00-0c-cd-cd-d0 are intercepted and their MAC destination address is overwritten to the MAC destination address used for the corresponding protocol (PVST, STP, RSTP). The type of the STP protocol can be derived from LLC and SNAP encapsulation.
In the egress direction, all STP PDUs received on all VPLS ports are intercepted and L2PT encapsulation is performed for SAP/spoke-SDPs configured as L2PT termination points. Because of implementation reasons, PDU interception and redirection to CPM can be performed only at ingress. Therefore, to comply with the above requirement, as soon as at least one port of a specified VPLS service is configured as L2PT termination port, redirection of PDUs to CPM is set on all other ports (SAPs, spoke-SDPs, and mesh SDPs) of the VPLS service.
L2PT termination can be enabled only if STP is disabled in a context of the specified VPLS service.