Obviously you know these subnets overlap.
You cant configure overlapping addresses on a cisco. As above, if both networks are advertised onto a network that has a VLSM routing protocol, then the most specific route will win. Older routing protocols will not allow the variable length subnet mask so traffic will go to the closer router.
172.30.0.0 255.255.255.0 would have devices with an IP of 172.30.0.1 to 172.30.0.254.
172.30.0.0 255.255.252.0 would have devices with an ip range of 172.30.0.1 to 172.30.3.254 .
So the address you mention, 172.30.3.15 , only works on the second network. If the first network had an address of 172.30.3.0 255.255.255.0 then you could have the address on both networks, on two different routers.
Traffic would go to the more specific network, and the two devices would be unable to communicate. Especially since they would not send data to thier own ip address outside of thier own interface.
Hosts on the 172.30.0.0 255.255.252.0 network will not send traffic to the router trying to get to a device in this subnet, they will attempt to arp for that address. If you turn on proxy arp perhaps the router will route the packet. You can also put in static arps and host routes to do all sorts of foolish things.
Can't see why though.
