You have a network that has three subnets. You migrate the network to a Cisco ACI fabric. You want to bridge the three subnets without changing the IP addresses of the endpoints. The default gateway…

Creating multiple bridge domains would segment the subnets, not bridge them into a single flood domain, and unicast routing disabled means ACI would not route between them if needed.

While a single bridge domain is correct for a single flood domain, disabling unicast routing would prevent ACI from acting as a Layer 3 gateway if needed, and it's generally best practice to enable unicast routing for proper subnet management in ACI even if the gateway is external.

To bridge existing subnets without changing endpoint IPs and with the default gateway outside ACI, a single bridge domain should be created. Unicast routing must be enabled on this bridge domain to allow the fabric to learn and forward traffic for the subnets, even if the gateway is external, and to support potential future routing within ACI. Corresponding subnets within this BD represent the existing subnets being bridged.

Creating multiple bridge domains would separate the subnets into different broadcast domains, not keep them in a single flood domain.