After further research, this appears to be https://github.com/FRRouting/frr/issues/5146. As suggested there I have set net.ipv4.udp_l3mdev_accept=1 and that works.

I'm puzzled though, it seems like https://github.com/FRRouting/frr/pull/5274 should have opened the VRF socket without needing the sysctl?

I'd be tempted to test with a newer kernel.

-- 
Tim Bray
Technology Director, ProVu Communications Ltd, Huddersfield, UK.
http://www.provu.co.uk/     Phone: +44 1484 840048
IP Telephones, Drop shipping, pre-configuration, XML orders
*****    If it is important, just phone    *****