This feature allows, at the VPLS instance level, MAC subnetting, such as learning and switching based on a configurable number of bits from the source MAC address and from the destination MAC, respectively. This considerably reduces the VPLS FDB size.
MAC scalability involving MAC learning and switching based on the first x bits of a virtual MAC address is suitable in an environment where some MAC addresses can be aggregated based on a common first x bits, for example 28 out of 48. This can be deployed in a TPSDA environment where the VPLS is used for pure aggregation (there is no subscriber management) between the DSLAM and BRAS devices. The DSLAMs must be able to map customer MAC addresses to a pool of internal virtual MAC addresses where the first bits (28, for example) identify the DSLAM with the next 20 bits identifying the DSLAM slot, port number, and customer MAC station on that port. The VPLS instances in the PE distinguishes only between different DSLAMs connected to it. They need to learn and switch based only on the first 28 bits of the MAC address allowing scaling of the FDB size in the PE.
Figure: Subnetting topology displays a Layer 2 PE network (such as the ESS-Series) aggregating traffic from DSLAMs (Nokia) to BRAS devices. The VPLS service is running in the PEs directly connected to the DSLAMs (VPLS PE1) while the PEs connected to the BRAS devices are running a point-to-point Layer 2 service (Epipe).
Nokia DSLAMs have the capability to map every customer MAC to a service provider MAC using the virtual MAC addressing scheme depicted in Figure: VMAC subnetting topology.
As the packet ingresses the DSLAM from the residential customer, the source MAC address (a customer MAC for one of its terminals or routers) is replaced by the DSLAM with a virtual MAC using the format depicted in Figure: VMAC subnetting topology.
The U/L bit is the seventh bit of the first byte and is used to determine whether the address is universally or locally administered. The U/L bit is set to 1 to indicate the address is locally administered.
The following bits are used to build the VMAC address: DSLAM ID bits 39 to 21, slot ID bits 20 to 15, port ID bits 14 to 6, and the customer station ID bits 5 to 0.
Based on this scheme, it is apparent that the VMACs from one DSLAM have bits 47 to 21 in common.
The VPLS instance in PE1 only learns the first part of the MAC (bits 47 to 21) and, as the packets arrive from the BRAS device, switches based only on these bits in the destination MAC address to differentiate between the connected DSLAMs. When the packet arrives at the DSLAM, the entire destination MAC is checked to determine the slot, port and which specific customer station the packet is destined. As the packet is sent to the customer, the DSLAM replaces the destination MAC address with the actual customer MAC corresponding to the customer station. The following are VPLS features not supported when the VMAC subnetting feature is enabled:
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Blocked features (CLI consistency checked provided)
Residential Split Horizon Groups
BGP AD
TPSDA (subscriber management) features
PBB
VPLS OAM (MAC populate, MAC ping, MAC trace, CPE Ping)