Thanks for all your comments.
The thing is , I have tried putting IPV6INIT=yes , IPV6_AUTOCONF=yes and DHCP6C=yes in the respective ifcfg-eth* scripts. But still dhcp6c client is not getting started once I start rcnetwork. I have to start it separately.
For SLES11, there BOOTPROTO=dhcp means it will start both the clients once we start the network. There I did not face any issue.
But here it is only identifying the flag related to dhcp ipv4 only.

Please advise.