You can configure a static route on the tier-0 gateway to external networks. After you configure a static route, there is no need to advertise the route from tier-0 to tier-1, because tier-1 gateways automatically have a static default route towards their connected tier-0 gateway.
Recursive static routes are supported.
Procedure
What to do next
Check that the static route is configured properly. See Verify the Static Route on a Tier-0 Router.