# ndd -get /dev/ip ip_forward_src_routed
if the value is 1 then the machine will forward source routed frames, if 0 it wont. 
ndd -set /dev/ip ip_forward_src_routed 0
docs.sun.com has a Solaris Tunable Paramters Refrence manual which is pretty handy take a look at that.

Some other useful info: 

# ndd /dev/ip ?

This is covered in detail in the BluePrint OnLine article 'Solaris OE Network Settings for Security: updated for Solaris 8 OE' pages 15 and 16. The article is available at:

   http://www.sun.com/blueprints/1200/network-updt1.pdf

There is a script (nddconfig) available from http://sun.com/security/blueprints which can be used to automate setting this option and others