L2PT is used to transparently transport protocol data units (PDUs) of Layer 2 protocols, such as Cisco Discovery Protocol (CDP), Dynamic Trunking Protocol (DTP), Port Aggregation Protocol (PAGP), Spanning Tree Protocol (STP), Unidirectional Link Detection (UDLD), VLAN trunking protocol (VTP), and Link Layer Discovery Protocol (LLDP). This allows users to run these protocols between customer CPEs without involving backbone infrastructure.
The 7210 SAS routers support the 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, rather than via direct connections. In this type of environment, the termination of tunnels through the infrastructure is required.
L2PT tunnels transport 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. On egress of the tunnel, the MAC address is overwritten back to the MAC address of the respective Layer 2 protocol.
The 7210 SAS nodes support L2PT termination for STP BPDUs as follows:
On ingress of every SAP or spoke-SDP that is configured as an 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. The type of protocol can be derived from LLC and SNAP encapsulation.
In the egress direction, PDUs of the corresponding protocol received on all VPLS ports are intercepted, and L2PT encapsulation is performed for SAP and spoke-SDPs configured as L2PT termination points. For implementation reasons, PDU interception and redirection to CPM can be performed only on ingress. Therefore, to comply with the preceding requirement, as soon as at least one port of a specific VPLS service is configured as an L2PT termination port, redirection of PDUs to CPM are set on all other ports (SAPs, spoke-SDPs) of the VPLS service.
L2PT termination can be enabled only if STP is disabled in the context of the specific VPLS service.