In my router there are 2 fastethernet.
The fa0/0 is 10.100.253.253/16 and is connected in a linux server with 2 NIC, a with 10.100.253.254/16 and other with 10.0.100.120/16 with various host behind.
My qyestion is. Which is the difference between:
ip route 10.0.0.0 255.255.0.0 fastethernet0/0
ip route 10.0.0.0 255.255.0.0 10.100.253.254
10.0.0.0 255.255.0.0 fastethernet0/0 --> When this static route is used the router would ARP out fastethernet0/0 int for any packet that has a destination address of 10.0.x.x. Unless the device (linux server) can proxy ARP on behalf of the host (destination address) this packet would fail.
As you can see this kind of static route should be avoided unless there's no other option. Instead of the linux server if you had a Cisco router it would have responded with proxy ARP on behalf of the destination host and pings would have succeeded. Proxy ARP has some other downside as well it can high CPU utilization on the router that does proxy ARP, high number of broadcasts (ARP) on the LAN etc.
With that said you are better off always pointing a static route to a next hop address rather than the egress interface to avoid these problems.