Need help with configuring large packet size on Solaris 7 / e6500

We're running Solaris 7 on FDDI n/w on an E6500 host and wish to use MTU (packet size) > 1500, more like 3072 bytes to begin with and possibly up to 4096 bytes.

Linux has /etc/network/interfaces. Does ANYONE remember the equivalent in Unix? When I do ifconfig eth0 mtu 4000, I get the error SIOCSIFMTU: Invalid argument

Want to know what is the file (or files) to modify and what are the values to add or modify.

Thanks in advance