When I set my ospf interface eth1's address to be 10.2.1.2/30, neighbor router ethernet interface IP address to be 10.2.1.1/30, the neighbor shows it's PTP type:

bird> sh ospf neighbors

ospf_rdomain_0:

Router ID Pri State DTime Interface Router IP

172.16.20.76 1 Full/PtP 00:35 vni-0 10.2.1.1

If I change the interfaces' IP address to be 10.2.1.x/24 or I force interface type to be bcast in Bird's ospf config, it shows as

bird> sh ospf neighbors

ospf_rdomain_0:

Router ID Pri State DTime Interface Router IP

172.16.20.76 1 Full/DR 00:35 vni-0 10.2.1.1

My question is how does Bird detect the connected network type?

Thanks,

Wei