I have to make the 192.168.1.* network inside the already existing 192.168.0.* network using a Cisco 2821 router.

Network diagram

I'm not familiar with Cisco IOS, and I have no idea what commands to use and how to make this whole thing work.

Also, the router provided by the ISP doesn't support static routes, and they don't have any other models that do, so any solution that needs the ISP's router to support static routes won't work. It's not possible to get rid of the ISP's router, because it is a DOCSIS 3.0 modem as well. It's not possible to disable the router part of it and use it as modem only. Tried looking for a DOCSIS module for the Cisco 2821, but the latest one is only DOCSIS 2.0, which is inadequate. Replacing the router/modem combo with a modem only box isn't possible either; the ISP said that it only works with the gear they supply.

  • 1
    If you want this to work you’ll have to wire your Internet through the Cisco router Jun 25, 2018 at 22:30
  • That's not possible, because the ISP's router is a DOCSIS modem as well. Jun 25, 2018 at 22:33
  • The ISP modem has an ethernet interface doesn’t it? Just use the modem wired to the Cisco Jun 25, 2018 at 22:34
  • The ISP's router is a DOCSIS modem and a router in one "package", and as far as I know there's no way to turn off the router part of it and only use it as a modem. Internet comes in through a coaxial cable. Jun 25, 2018 at 22:36
  • You should be able to buy a compatible cable modem that is not also a router. The cable ISPs that I know of have a list of compatible cable modems. You will need to give the ISP some information off the cable modem you get. Your business should own its own equipment, anyway.
    – Ron Maupin
    Jun 25, 2018 at 22:38

1 Answer 1



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:

                                                   Gi0/0         Gi0/1

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 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.

  • I have an IEEE802.1Q capable switch, and the 2821 only has 2 interfaces. Jun 27, 2018 at 4:06
  • Just so I know whether to give you switch configs as well, is it a Cisco IOS switch, or something else? Jun 27, 2018 at 11:15
  • It's an SMC GS24C-Smart switch Jun 28, 2018 at 11:00

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