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Will the network configuration below work?

[ 8-port Managed Switch ]

  • VLAN_1: Port 1 = Cable Modem
  • VLAN_2: Port 2 = Router WAN
  • VLAN_3: Ports 3-8 = Other

Can the router on VLAN_2 broker between the cable modem on VLAN_1 and other wired devices on VLAN_3?

I have an obihai which connects to a phone line and another cable tv device which brokers cable outlet to the cable modem as well as plugs into an ethernet port on the WRT54GL's router switch. The phone line and cable outlets are both next to each other. The problem is that the WRT54GL's signal degrades greatly in that location.

So, currently, I have a WRT54GL doing NAT, DHCP and routing between WAN and the obihai and cable tv device as well as an RT-AC68U wireless router which is connected via a powerline to a central part of the home and configured as a client bridge which all wireless (and other wired) clients connect to.

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  • As of writing, I haven't used one, i'd guess that it can, and this is just an idea, but perhaps it'd work if the router is in some way a member of both VLANs.. i.e. if both VLANs include Port 2. i'm sure many here will know as it's a straightforward problem.. I've heard about switches with VLANs and no built in router, having the issue that VLANs can't communicate but they can when you attach a router. so i'm sure it's a pretty standard operation. – barlop Dec 12 '14 at 0:28
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This may vary depending on your ISP, but generally it will work if you do the following.

The cable modem's Ethernet interface and the router's WAN or external interface need to be able to communicate. So both Port 1 and Port 2 should be in the same VLAN.

Now if you can connect one of your router's LAN or internal interfaces to the switch in a separate VLAN shared with the other devices, then things should work fine.

However based on your description, it almost sounds like you want your wireless separate from everything else that is connected and are planning to use a powerline bridge. This will be difficult if you need two network connections between your modem/switch and router. So what you may want to do is to look for an access point (or a consumer router that can be configured as an access point) to locate away from the other devices, which will only require one network connection and leave the device performing as your router by the cable modem (with or without the wireless disabled).

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  • i'm not that familiar with what he is doing but since you're suggesting he adjusts his VLANs anyway.. What if he put the router in the same VLAN as his cable modem(as you say), but then also put in every VLAN. If he wants every VLAN to be able to access each other and go on the internet then he could do that. – barlop Dec 12 '14 at 20:12
  • @barlop, problem with that approach is that unless the cable modem is providing gateway services (i.e. DHCP, NAT, etc), the ISP is generally only providing one IP address for use. Many cable modems will just bridge the connection and the router will get the one IP address. Even if the able modem is providing gateway services, it may not be providing the security benefits of what the router may provide. Of course, situations vary... – YLearn Dec 12 '14 at 23:30
  • i'm familiar with NAPT. When I say modem I mean bridging, and router doing NAT (so no double NAT and so the better device doing NAT). I am less familiar with VLANs. I still don't see why not either A)plug the cable modem into the router and set the port the router in connected to, to be in every VLAN. or B)Put router and modem in same VLAN(as you say) but put the pair of them in every VLAN – barlop Dec 14 '14 at 1:26
  • VLANs separate a physical switch into several logical switches. In case "B", you wouldn't put a switch between the modem and router and then connect all other devices to that switch (i.e. outside the router). In case "A", the reason you may want to use a managed switch can be numerous. A managed switch may provide better statistical data. It can provide the ability to span a port outside of the router to capture traffic without having to disconnect and insert a tap. ou may be able to use certain security features of the managed switch. It all comes down to what the OP is trying to do. – YLearn Dec 14 '14 at 16:00
  • I am not asking why he should use a managed switch. And, regarding putting a switch between modem and router.. I am not sure whether you mean the VLAN aware device or some new switch.. so i'm not sure if you understnnd me right. But, looking at his question, he has the router and modem on two different VLANs which is effectively like having a switch between them, which is probably wrong. I am not suggesting having a switch between them. In Case A, I suggest having the modem plugged into the router so no switch between them... – barlop Dec 14 '14 at 17:26

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