Using EPG extension, an engineer has moved all endpoints in a VLAN into an ACI fabric. When he moves the default gateway from traditional switches into the fabric, he suddenly loses all connectivity…

If the L2 connection between ACI and non-ACI switches was disconnected, it would typically affect all connectivity, not specifically the loss of external connectivity when the default gateway is moved into the fabric and the issue is more about L3 routing and policy.

When a default gateway for a subnet is moved into the ACI fabric, the fabric becomes responsible for routing traffic for that subnet. If there is no Layer 3 Outside (L3Out) configured and associated with the Bridge Domain (BD) that hosts this subnet, the fabric cannot learn routes to external networks or advertise its own subnets, causing a loss of connectivity to endpoints from outside the fabric.

For a subnet configured within a Bridge Domain in ACI to be reachable from outside the ACI fabric, its "Scope" parameter must explicitly be set to "Advertised Externally". If this setting is not enabled, the fabric will not advertise the subnet to external routing protocols via an L3Out, preventing external devices from routing traffic to it.

Cisco ACI uses contracts to define permissible communication between EPGs within the fabric and between EPGs and external networks via an L3Out. If endpoints are in an EPG and the default gateway has moved into the fabric, but no contracts are in place between that EPG and the L3Out, then communication from outside the fabric to these endpoints will be blocked by ACI's policy enforcement.