sb5120 and gigE nic

Hello All,

First time posting here.
Has anyone had any troubles with an sb5120 negotiating with an gigE nic?
Here is what I am seeing. The new machine will work/get an ip address from the ISP correctly,providing a Linksys SOHO router is in between the sb5120 and the linux server. If I take the Linksys out and plug the sb5120 directly into the gigE nic of the Linux server, I do not even get an amber solid light/activity ,until I try and activate the nic,once this is done then the activity light does start showing activity. . The machine never does get an ip address this way. I am wanting to eliminate the Linksys piece as the Linux machine is acting as the house router ( It is a terminal server setup). Also I have tried an cross over cat5 as well just to try something to get an solid light as soon as the cat5 wire is plugged into modem. This modem has the original firmware I m sure.
Is it time to go DOCSIS 3.0?

Thank You,
Barry