It should be quite easy to connect 2811 FastEthernet to cable modem. I have not done it specifically on the 2811 but I have configured many 1841s with FastEthernet connection to a cable modem and they work well.
I thought the 2 fixed FastEthernet ports were for the connection between the router and the LAN and it was through the interface cards (T-1, cable, etc.) that you connect the router to the Internet. Are you suggesting that FE port can be used to connect to the Internet, i.e. can it be programmed as such inside the router or are you suggesting that it is simple to get a FastEthernet interface card?
You should be able to tell the Ethernet port connected to the cable modem to acquire its interface address from the modem using DHCP.
You might need to then NAT, if the modem only expects one device.
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