Tjyylli

So I have an Arris Technicolor TG2472 cable modem at home provided by my ISP. The modem has a WAN IP A.A.A.A and an internal IP address of 192.168.0.1 that is for the default-gateway. The modem device also has 4 Ethernet ports. Connected to one of the Ethernet ports I have a Cisco 2911 ISR router, so it's modem port E1 to 2911 g0/0, g0/0 IP = 192.168.0.200. The 2911 port g0/1 is connected as a trunk port to a Cisco 3750 switches g4/0/2 port. This switch defines multiple vlans and the 2911 router provides inter-vlan routing via it's sub-interfaces g0/1.10, g0/1.20 and g0/1.99.

From the router, I am able to successfully ping the cable modems gateway address of 192.168.0.1. However I am not able to ping that same gateway address from any device on one of my vlans, and I am also unable to reach the internet from those devices, although, I am able to ping other devices on the same vlan.

The routing table on my 2911 contains routes to all vlans and also the 192.168.0.1 network. as well as a default route sending traffic to the modems gateway. I believe that my problem comes from the cable mode not having a route back to any of the vlans and being unaware of their existence. So I think that when a device on a vlan sends a ping or a web request that goes across the router into the 192.168.0.0/24 network everything goes fine until, on the return trip, the cable modem sees a packet destined for an address (e.g. 172.16.0.43/24) that belongs to a device on one of the vlans and thinks because this address is not on 192.168.0.0/24 it should send the packet out the WAN link.

This cable modem does not allow me to add a static route (I don't believe that's really a modem feature is it?) and so I am wondering if there is some other device that could be used as a replacement (my service is provided through Coaxial cable from COX cable) or some type of workaround to allow my 2911 to act as the router for both networks

This cable modem does not allow me to add a static route (I don't believe that's really a modem feature is it?)

Well your "modem" isn't really just a modem either, is it? It has a WAN address and a LAN address ("for the default-gateway"), and you said that your Cisco 2911 even uses it as a gateway – it is therefore a router. (This doesn't stop it from also being a modem, of course.)

That said, if it were only a modem: yes, it's not a modem feature, but it is a host feature, to some extent. (For example, major PC operating systems use a full-featured routing table even when acting as end hosts.) When you connect to the modem for management, it is acting as a host – so it could use a routing table.

I am wondering if there is some other device that could be used as a replacement (my service is provided through Coaxial cable from COX cable)

Cable modems use the standard DOCSIS protocol for the coaxial cable link. COX currently allows using your own modem instead of the ISP-provided one:

• https://www.cox.com/residential/support/cox-certified-cable-modems.html


• https://www.cox.com/residential/support/self-activating-your-new-cox-equipment.html

I am wondering if there is [...] some type of workaround to allow my 2911 to act as the router for both networks

There are several options:

• Configure the 2911 to apply SNAT (masquerading) for all traffic to 192.168.0.1. Since the modem will see the client pings as coming from 192.168.0.200 (local to it), it will reply through the correct interface.




• If the modem was only acting as a modem and not a router, you could have the inverse relationship, i.e. have the modem use your 2911 as its default gateway.




• If the two networks were numbered close to each other (e.g. 192.168.0.0/24 and 192.168.1.0/24), you could configure the modem with a larger subnet mask and use proxy-ARP to make it think the client addresses are local.