Paul L on Nostr: If the upstream router was running a dynamic routing protocol like OSPF then you ...

If the upstream router was running a dynamic routing protocol like OSPF then you could run that on the physical host also and use that to announce routes for the virtual subnets out of eth0 so that the physical network knows about them

For IPv6, prefix delegation can do most of that and without needing OSPF but it is not specific to virtualisation, and still involves the physical host being a router.

IPv4 NAT doesn't do this only because it pretends everything comes from the eth0 address.