I have a problem between my 1750 router with Fasteth interface, and a 16 port Hub (3 Com) I set the speed to 10MB and half duplex.
The PC's connected to the switch can ping the router fasteth interface. When the entry in the arp table times out, the PC's cannot ping the router anymore. From the router you cannot ping the PC anymore either. When I clear the arp cache on the router, I can ping again. The default arp timer is set to 4 minutes.