L3Out Subnet Scope Options in Cisco ACI

L3Out Subnet Scope
Options in Cisco ACI

Scope Option	Purpose
Export Route Control Subnet	Advertises specific transit routes out of the ACI fabric. Used for controlling which external routes are exported.
Import Route Control Subnet	Allows specific external routes to be imported into the ACI fabric using BGP or OSPF.
External Subnets for External EPG (Security Import Subnet)	Enables data plane traffic between external EPGs and internal EPGs. Without this, traffic is dropped even if routes are learned.
Shared Route Control Subnet	Used in inter-VRF route leaking. Marks subnets that should be advertised across VRFs.
Shared Security Import Subnet	Similar to External Subnets, but for shared L3Outs. Enables traffic flow between shared external and internal EPGs.
Aggregate Export/Import/Shared Routes	Used to summarize routes, e.g., adding /32 in front of 0.0.0.0/0 for default route aggregation.

🧠 Key Notes

These scope options are critical for controlling
route advertisement and enabling secure communication between
different parts of the network.
Cisco ACI uses a contract-based
model, so even if routes are advertised, traffic will be dropped
unless explicitly allowed by contracts and security prefixes