Using Catalyst 3550 Switch with Linksys Home Router and Cable Internet

Question

I've about pulled what little hair I have out of my head on this one, and need some configuration help.
I have a Cisco Catalyst 3550 switch with five Windows 7 desktops, an Avaya PBX and five Avaya IP phones attached.  All of these devices are on a 192.168.0.0/24 subnet, and are communicating properly.  I will refer to this as network # 1. I also have SEPARATE network, we'll call network # 2, using AT&T ADSL service and a Netgear 4-port/wireless router/ADSL modem combo device, which is functioning properly with a couple of other Windows 7 desktops over its own wired Ethernet network, using DHCP, and also on a 192.168.0.0/24 subnet.  I thought it would be a simple integration, just plugging one of the 3550's ports to one of the DSL router's ports, in order to give the five Windows 7 desktop computers on network # 1 internet access via the DSL modem. Guess I was wrong.  When I connect the two switches together, although I get a good connectivity (green lights on both ports) and am able to ping the DSL router's gateway address (192.168.0.252) from network # 1's computers, the computers on network # 1 cannot access the internet. Also, the working computers on network # 2 lose their internet access as long as the two switches are connected together. I am not a Cisco guru, but there's got to be a way to make this scenario work.  Can someone provide me with a 3550 configuration that will allow me to extend my internet service from network # 2 on the DSL router to my 3550 switch and their computers?  Here's what I am looking for:
INTERNET ---> ADSL MODEM ---> NETGEAR ROUTER ---> CISCO 3550 SWITCH ---> NETWORK DEVICES WITH INTERNET ACCESS

Answer 1

The Netgear router is probably what's doing the natting. Is the 3550 configured for routing or is it straight L2? If you have the 3550 configured as L3, then it's going to be easy to do what you want. Just add a static route on the Netgear to point the subnet that it doesn't know about to the 3550. For example, if the Netgear is addressed at 192.168.1.1 and the Cisco 3550 is addressed at 192.168.1.2, but it also knows about the 192.168.0.0/24 (separate vlan), then you would put a static route on your Netgear for 192.168.0.0/24 to go to 192.168.1.2.
The way that I would do it is to create a separate vlan on the 3550 and assign an address to it. Once you do that, make the port that the other switch connects to an access port of that vlan. (It would need to be on the same subnet as the existing equipment.) All of your devices would use it as a default gateway and then you would do the rest as above. You could also use RIP between the Netgear and Cisco if you can't do static routing.
HTH,
John