how the routers in ospf network will discover the borders routers
when i try to see the border routers with the show ip ospf border command
i get :
in Area 0 - R2 discover : all border routers in the network ( R3 - R5 - R8 - R12 )
in Area 1 - R4 discover : R3 - R5 - R12
in Area 2 - R6 discover : R5 - R8 - R12
in Area 3 - R9 discover : R8 - R12
my question is why all areas know border R12 ? how the routers discover the border router ?