In order to do this, you will have to create a startup script. The standard way this is done in Solaris is as follows:
1. create the script in /etc/init.d
2. create a link to the script in /etc/rc?.d
(the ? is for the runtime that you want the script to start in).
ie.
you may create a script called routes in /etc/init.d that may look something like this:
#!/sbin/sh
# Add the route to network
route add -net ... -nostatic
and then in /etc/rc2.d you would run the following:
# ln /etc/init.d/routes S98routes
the S98 specifies that this is a start script, and that the priority is 98 (in other words, it will happen after the scripts that have a lower number.
This would add the routes whenever the system boots into boot level 2 or greater, which would be my guess as to where you would want the script.
I hope this helps,
Chad Rytting