I've recently had a laugh on a UDM trying to setup IPv6 routing. Somehow, it did not install the default route in the FIB, but the OS was aware of it, so the router was reachable from the outside but did not route packets. I tried adding a route to `::0/0` and it spat at me that a multicast destination was not valid as a route destination. I gave it a route to `::0/1` and it's happily chugging along now. /shrug