Most of my network is based on Mikrotik routers and they serve me well. Even though I have decent mix of different operating systems present and I never had any issues. Until I got Ubuntu 19.10 Server up and running.
At the first glance, all worked perfectly and then suddenly machine would stop responding. Quick check from console interface pointed toward the culprit - there was no IP address present. Check on Mikrotik for DHCP leases was showing "waiting" state. Manually running dhclient on Ubuntu did temporarily resolve problem only to have address gone at the next DHCP expiration interval.
For some reason Ubuntu 19.10 Server would do DHCP once and then give up on further renewals. It was as if dhclient was crashing. Check of the syslog has shown curious entries:
/var/log/syslogsystemd-networkd: eno1: DHCPv4 address 192.168.1.103/24 via 192.168.1.1
systemd-networkd: eno1: Could not set DHCPv4 route: Network is unreachable
systemd-networkd: eno1: Failed
I had DHCP classless static route option set on my Mikrotik and it seems dhclient really didn't like my setup with multiple CIDRs. As soon as I removed all entries except for the default one, it started working properly.
On a hunch, I tried changing the network order to have default route as the first one and it worked. What also worked was omitting default gateway altogether and only having narrower CIDR definitions.
There is nothing in RFC3442 forcing this order and either ordering works for all other clients - including non-server Ubuntu 19.10. I guess something in Ubuntu Server configuration is making dhclient misbehave when things are not exactly how it wants it.
However, considering either ordering is perfectly valid, I decided to take a simpler route and update my classless route definition.