I'm not sure you understand what DHCP client means or how it works.
If you configure eth1, then the only setting is "DHCP". Once your interface is configured as DHCP client, it will broadcast, look for DHCP server, get IP address and other parameters... well everything required to properly configure eth1
Of course, this means that there is a DHCP server on this network segment, meaning your ADSL interface (router) is configured to provide DHCP server services
So back to your question: is or related to Zentyal or ADSL router configuration
If related to your router, then you should tell us a bit more...