Le van írva a dokisban minden, itt egy részeletesebb kifejtés:
https://netdevconf.info/1.2/papers/ahern-what-is-l3mdev-paper.pdf
De ha te szeretnél külön ip rule-okat minden VRF-hez megteheted:
https://www.kernel.org/doc/Documentation/networking/vrf.txt
2. An l3mdev FIB rule directs lookups to the table associated with the device. A single l3mdev rule is sufficient for all VRFs. The VRF device adds the l3mdev rule for IPv4 and IPv6 when the first device is created with a default preference of 1000. Users may delete the rule if desired and add with a different priority or install per-VRF rules. Prior to the v4.8 kernel iif and oif rules are needed for each VRF device: ip ru add oif vrf-blue table 10 ip ru add iif vrf-blue table 10