By default, which action does a bridge domain take with unknown unicast packets?

'1EP' is not a standard ACI term in this context; ACI uses the concept of endpoints and forwarding based on learned MAC addresses.

In ACI, if a leaf switch receives a unicast packet for a destination MAC address that is not learned in its local endpoint table or the spine proxy endpoint table, the bridge domain defaults to flooding this unknown unicast traffic across all associated ports. This behavior ensures connectivity until the destination is learned.

Forwarding to the gateway is for IP routing, not for unknown unicast MAC forwarding within a bridge domain.

The spine proxy is involved in learning and distributing endpoint information, but it does not directly receive and forward unknown unicast packets from a bridge domain in this manner; the bridge domain itself handles the initial flooding.