Let me see if I understand correctly - when you used another NIC and a crossover cable everything is working fine with 172.x.x.x?

If yes then could there be a switch issue or software installed on another system. We had a customer use GoldDisk on a system that would block certain requests from systems (like ping) that they forgot to mention. Please check for similar.

If no send logs to support. If you have sent logs please try to stay with one engineer for support this helps us duplicate efforts.