Comment 0 for bug 1768088

network:
  version: 2
  renderer: networkd
  ethernets:
    ens3:
      addresses: [192.168.3.30/25]
      dhcp4: no
      routes:
       - to: 0.0.0.0/0
         via: 192.168.3.1
         metric: 100
         table: 101

      routing-policy:
       - from: 192.168.3.0/25
         table: 101

    ens7:
      addresses: [192.168.5.24/25]
      dhcp4: no
      gateway4: 192.168.5.1
      nameservers:
        addresses: [1.1.1.1]

this doesn't activate a proper rule for table 101:
0: from all lookup local
32766: from all lookup main
32767: from all lookup default

in place of
0: from all lookup local
32765: from 192.168.3.0/25 lookup 101
32766: from all lookup main
32767: from all lookup default

the file in /etc/systemd/network contains the correct information:
[Match]
Name=ens3

[Network]
Address=192.168.3.30/25

[Route]
Destination=0.0.0.0/0
Gateway=192.168.3.1
Metric=100
Table=101

[RoutingPolicyRule]
From=192.168.3.0/25
Table=101

the ens3 is reachable from 3.0 but not from anywhere else

with ifupdown on another system both interfaces would be reachable from everywhere on the private subnets.
up ip route add default table 101 dev enp8s0 via 192.168.3.1
up ip rule add from 192.168.3.0/25 lookup 101
down ip rule del from 192.168.3.0/25
down ip route del default table 101 via 192.168.3.1