N+1 support for BNG M:N subscriber service redundancy

A single broadband network gateway (BNG) can perform as a backup BNG for multiple primary BNGs. You can configure the backup BNG in service-activation-on-failover mode, which consumes less resources to back up the maximum number of subscribers. The service-activation-on failover mode enables the line cards in the backup BNG to host three times more subscribers than the primary BNGs. This enhancement significantly reduces the resources that are reserved for redundancy in the backup BNG.