Once you know the correct CLI to add the route, you write a startup-script to run the command when the server boots.
To do so, first create the file/script by typing:
#touch /etc/rc3.d/S101addroute
(The S101addroute is just the name of the file/script).
Then:
#vi /etc/rc3.d/S101addroute
Using vi, just create a standard bash script:
#!/bin/bash
inet_route add XXX YYY ZZZ
Now, every time the ESX server boots (well, everytime it boots into runlevel 3), the script will run therefore creating the desired route.
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Under no circumstances are you to award me any points. Thanks!!!