Thought I had everything working. I had created a 2nd
bridge for use with virtual machines I want to keep
off my local lan (one of the VMs operating as a
firewall with two ethernets).
Then I rebooted after installing updates, and I couldn't
talk to anything in the outside world. Aaugh!
Finally discovered I had two default routes, one (the
one it picked for everything, of course) going through
the 2nd bridge to nowhere.
Eventually found magic incantation:
nmcli con modify bifrost ipv4.never-default true
(bifrost being my 2nd bridge).
I may eventually get used to network manager, or I
may die of old age.