After Multi-chassis Synchronization (MCS) for subscriber management and SRRP are enabled, both BNG nodes, Active (SRRP master state) and Standby (SRRP backup state) forward packets (for subscribers) in both directions.
Traffic flows through an SRRP enabled node according to the entries in the SRRP sync database and the SRRP state of the node:
SRRP in backup state directs downstream traffic over the redundant-interface toward the active node (SRRP master state). If the redundant interface is unavailable, traffic is sent directly to the subscriber.
SRRP in master state always directly forwards the downstream traffic toward the subscriber.
In the upstream direction, the active SRRP node accepts subscriber traffic addressed either to the MAC address of the SRRP active group OR the native interface MAC address.
The standby node accepts in the upstream direction only packets addressed to its native interface MAC address.
If both SRRP nodes become active (SRRP master state), then both forward traffic to or from subscribers unaware of the link failure somewhere in the Layer 2 network. As a result, downstream traffic can be blackholed. Whether downstream traffic is lost depends on the native routing on the network side, which is unaware of the failures in the aggregation network.