WAN and LAN over same wires

Question

My home is currently Ethernet wired via EOP (Ethernet over Powerline) units. I was wondering if I could send the WAN connection into one of the rooms and then have the LAN connection come back through the same wire.

Here is a drawing of what I want to do:

The problem is that the router have to be elsewhere then the modem (WAN). But then the EOP will transfer both LAN and WAN connections.

Will this set up work?

Answer 1

Alone, it won't work well.

For example: If your router uses pure DHCP on its WAN side (this varies between ISPs), and you "loop back" its LAN side into a WAN switch, well, you now have two DHCP servers on the same broadcast domain, and the router fighting with a bunch of DHCP clients. One of your other devices might end up claiming the WAN IP address before the router does. Or the router might end up assigning a "WAN" address from its own LAN pool.

---

One possibility of making it work is using VLANs (802.1Q tagging), which can be done by nearly all managed switches. You would have to assign the modem port to e.g. VLAN 10, regular device ports to VLAN 20, and EoP ports to 'trunk' mode (all VLANs, tagged). Likewise on the router's side – the "WAN" port would belong to VLAN 10, device ports to VLAN 20, EoP port 'trunked'.

^{(The example VLAN IDs were chosen arbitrarily out of the available 1–4095 range.)}

This however requires at least two switches to have the VLAN function ("Hub"? Really?), and it might get a bit more difficult if the router itself expects to be able to use multiple VLANs on the WAN/ISP side.

Also, in many cases EoP would have to carry the same data twice – once towards the router in VLAN 10, and once towards the device in VLAN 20, thus reducing usable bandwidth by half.

---

Your other, easier, option would be to move the router next to the modem. If this causes problems with Wi-Fi – buy a few dedicated Wi-Fi access points.