For every subnet that is configured under the L3Out, specific flags exist that are frequently used for in or out redistribution with external routers.
Some of the options might be greyed-out, which depends on the protocol that is selected from the top level of the L3Out configuration.
The Import Route control is only available for OSPF and BGP and has to be selected on the main External Routed Networks interface.
External Route Control Subnet
The External Route Control Subnet flag is used only when you want to export a transit route (a route that has been imported on an L3Out) through another L3Out.
This option does not have any effect in the ingress direction, therefore it can be utilized only when exporting routes.
The effect of this flag is to add an entry in the prefix-list, which is consumed by an outgoing BGP route map or ingress OSPF area filter.
This option has no influence on the routing table — it will not add a static or any other entry in the routing table or database. It is used only to filter subnets in the outgoing direction.
By default, all external routes are accepted and all outgoing routes are denied. The defined prefixes and assigned flag will allow the subnets to be advertised out.