Add's external_gateway_info to the provisioning router.
Using the undercloud as the default router is not-practical when nodes are not on the same L2 network as the undercloud.
Add a router for the public_net with external_gateway_info.
TripleO Controller nodes use the External network as the default gateway, so this network also need external gateway info so that these nodes can reach external resources such as ntp servers, dnsservers etc.
external_gateway_infoto the provisioning router. Using the undercloud as the default router is not-practical when nodes are not on the same L2 network as the undercloud.external_gateway_info. TripleO Controller nodes use the External network as the default gateway, so this network also need external gateway info so that these nodes can reach external resources such as ntp servers, dnsservers etc.