I'll say on the front-end that the best device to handle LAN routing between corporate subnets is not a Cisco 2821. A 2821 can do the job, it's just not really got the features you may want as you expand in this kind of scenario (namely, the ability to create SVIs, and a modern spanning-tree implementation). The Cisco 2821 router leaves you stuck with either legacy spanning tree (if using the switch expansion module), or a router-on-a-stick network as you expand past the interface density of the system
To set this up, you'll need a topology like this:
192.168.255.1/16 192.168.255.2/24 192.168.1.1/24
There are a few things to note:
- I assumed you only have the default Cisco 2821 interface configuration of 2 GigE ports. If you have three GigE interfaces in this router, leave a comment and I can redraw the toplogy.
- The subnet mismatch on Cisco2821:Gi0/0 is intentional, because you said that your Docsis modem does not support static routes. The simplest way to make your Docsis modem reach all subnets is to trick it into thinking your company has a flat /16 network. By default, Cisco IOS will proxy-arp for the machines connected to Cisco2821:Gi0/1 in this topology, so your modem will know how to reach all the subnets attached to Gi0/1.
- Gi0/1 could be configured one of two ways, depending on the kind of switch you have. If you have a dot1q-capable switch, then you can configure Gi0/1 with a real dot1q trunk. Otherwise, you need to configure Gi0/1 with 192.168.1.1/24 as a secondary address.
So to summarize:
- Tell me whether you have a dot1q-capable ethernet switch
- Tell me whether you have more than two GigE interfaces on the Cisco 2821 (there are pluggable expansion modules that you can buy seperately).
I'm not familiar with Cisco IOS, and I have no idea what commands to use
and how to make this whole thing work.
After you provide the information requested above, I can help with the Cisco IOS commands.