FAQ - Setting up Linksys with RCN One-way cable modems
specifically the 3COM CMX modems. This may apply to the 2 way networks also. But I'll never have that here.

This is an FYI after lotsa trial and error:

I recently upgraded from RCN Hybrid Cable Modem
(14.4 upload modem)to the 3COM CMX Cable Modem with
a 56K external modem upload. These are both one-way.

FYI: RCN's new system is requiring that you set your modem to emulate the address of the the network card used in the original setup of the modem. They won't tell you this and I don't know if the linksys techs can trouble shoot that, but if you don't do it, you won't be able to authenticate on the next time you dial up and hence lose your service until the modem is rehooked to the original PC and reconnected.

If you have the latest Linksys BIOS you can go to advanced and set the network MAC to the same as your card (found by using winipcfg & clicking advanced.)

Hope that saves someone else the time and misery I had this weekend.