First, are you adding the 172 address as the only address (getting rid of the 129)?

Second - run uname -a - it should tell you what version of Solaris you are running.

Third - look to see if you have an /etc/hostname.* file - see if the * portion is an interface.

Post the info you gather back here. Also take a look at Configuring TCP/IP on the Network (for Solaris 2.4)