How to do external loopback across NIC ports in same machine


I have a server setup with 2 NICs’ each with 2-4 Ethernet ports. I need to setup an external loopback using a cross-over cable connecting 2 different Ethernet ports (let’s say eth0 and eth4) on the same or different NICs’ for diagnostics
purposes. The internal loopback through the kernel loopback device (‘lo’) must be disabled and packets sent to either eth0 or eth4 as mentioned above must be forced out through the cross-over cable to the port on the other side of the cross-over cable. So for example a ‘Ping’ of eth0 ip address from eth4 ip address (using the -I option) would succeed even without the internal loopback (‘lo’ forced down) as long as both those ports are connected using a crossover cable. The ‘Ping’ would fail if the cable is disconnected.

What I find is that the corss-port Ping works only if the internal loopback device (‘lo’) is enabled and the external cross-over cable in that case has no effect on Ping when it is disconnected!

I saw some emails hinting a method to use “crossed route entries” to achieve this; albeit without actual illustrations. I tried the route command to create routes for ip’s assigned to eth0 and eth4 to reference eth4 and eth0 respectively, however it did not work.

Thanks a bunc in advance for your help.