ospfd(8), ospf6d(8)

Configuration:

router-id 192.0.2.22
redistribute default set { metric 200 type 2 }
area 0.0.0.0 {
        interface ix0 { metric 5 }
        interface ix1 { metric 5 }
        interface ix2:192.0.3.1 { passive }
}

To announce a route, it needs to be in the FIB:

$ doas /sbin/route add -inet 0.0.0.0/0 127.0.0.1 -blackhole