L2-Aware NAT allows overlap of inside (private) IP addresses between Enhanced Subscriber Management (ESM) (or L2-Aware NAT) subscribers. For example, IP addresses assigned to hosts within, for example, subscriber SUB-1, can be identical to IP addresses assigned to hosts within, for example, subscriber SUB-2. This is possible by the subscriber-ID field (which must be unique in the system) that is a part of the NAT translation key. This way the return traffic (in downstream direction) belonging to different ESM subscribers with overlapping IP addresses can still be differentiated by a unique ESM subscriber-id field that is used in reverse NAT translation.
L2-Aware bypass functionality with a failed MS-ISA breaks the logic because traffic is not translated (NAT’d) in SR OS node, and therefore, the return traffic does not take subscriber-id field into forwarding consideration. For this reason, the overlap of inside (private) IP addresses between ESM subscribers is not supported by the L2-Aware bypass functionality for the routed traffic within the same inside routing context. In other words, private IP addresses must be unique across the subscribers within a specified inside routing context.