Figure: L2-Aware tree shows the L2-Aware tree.
NAT is supported on DHCP, PPPoE and L2TP. Static and ARP hosts are not supported.
In an effort to address issues of conflicting address space raised in draft-shirasaki-nat444-isp-shared-addr-02, an enhancement to Large Scale NAT was co-developed to give every broadband subscriber their own NAT mapping table, yet still share a common outside pool of IPs.
Layer-2 Aware (or subscriber aware) NAT is combined with Enhanced Subscriber Management on the 7750 SR BNG to overcome the issues of colliding address space between home networks and the inside routed network between the customer and Large Scale NAT.
Layer-2 Aware NAT allows every broadband subscriber to be allocated the exact same IPv4 address on their residential gateway WAN link and then proceeds to translate this into a public IP through the NAT application. In doing so, L2-Aware NAT avoids the issues of colliding address space raised in draft-shirasaki without any change to the customer gateway or CPE.
Layer-2-Aware NAT is supported on any of the ESM access technologies, including PPPoE, IPoE (DHCP) and L2TP LNS. For IPoE both n:1 (VLAN per service) and 1:1 (VLAN per subscriber) models are supported. A subscriber device operating with L2-Aware NAT needs no modification or enhancement, existing address mechanisms (DHCP or PPP/IPCP) are identical to a public IP service, the 7750 SR BNG simply translates all IPv4 traffic into a pool of IPv4 addresses, allowing many L2-Aware NAT subscribers to share the same IPv4 address.
More information about L2-Aware NAT can be found in draft-miles-behave-l2nat-00.