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A TP-Link W8980 acts as a DHCP server on 192.168.0.0/24 subnet. This is also the only wifi access point.

A Zyxel USG 50 is connected to it through its 2 WAN ports and has static addresses 192.168.0.253 and 192.168.0.254

The Zyxel also acts as a DHCP server on 192.168.1.0/24 for the devices connected through its 4 LAN ports.

Is there any way to make the Zyxel the only DHCP server and create a unique 192.168.1.0/24 subnet, also for the devices connected to the TP-Link via wire or wifi?

I already tried to bridge WAN1 and LAN on the Zyxel and this way I can make the TP-Link to act as the only DHCP server, but this is not exactly what I want (and, furthermore, I lose the possibility of doing traffic balance on the 2 WAN ports of the Zyxel)

EDIT: I've made this simple image to better explain the situation. The desired result is that all the devices (wired or wifi) are on the same subnet

Desired result

Michele Scuttari
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2 Answers2

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The typical configuration for a single DHCP serving multiple subnets is to setup the subnets as routed (usually they are VLANS) and activate DHCP-Relay (helper-address or what else) on the device(es) working as routers.

If guess that the 192.168.1.0/24 network is behind NAT and not routed. Moreover I suppose that your network devices are "unmanaged" and don't support any of the needed features.

I think you cannot do that using your network equipment.

Maybe the guys at SuperUser, who deal with consumer level devices, could give you some tip.

EDIT:

Trying to guess, since I don't have any hands-on experience on your equipment:

  • disable NAT on the Zyxel (or connect the Tp-Link on a Zyxel LAN port)
  • setup two VLANS and assign the ports on both devices (Zyxel and TPLink)
  • Assign Ip Addresses to the VLANS on the Zyxel; check if you need to enable routing and/or setup specific policies
  • set up a Default Route on the Zyxel (something as 0.0.0.0/0 using the TP-Link as next hop)
  • you should not have to add any static route for the two VLANS on the Zyxel since both are "connected"
  • depending on your configuration, you will have to add some route to the TP-Link, informing it that the 192.168.1.0/24 network is behind 192.168.0.XXX (ip address assigned to the 192.168.0.0/24 VLAN)
  • setup DHCP for the two networks on the Zyxel
  • disable the DHCP Server on the TP-Link
  • configure the DHCP relay on the TP-Link (--> 192.168.0.XXX, see above)
  • cross your fingers

I would move the Zyxel at the border of your network, using it as Internet Gateway (instead of the TP-Link) and use the TP-Link as a bare access point. Different configuration: it would be cleaner.

Px2016
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  • Thank you for your answer. The Zyxel is a semi-professional device which I bought on ebay to make some practice. On the TP-Link router I have the possibility to set static routes, manage VLANS and set the DHCP relay. Obviously the same is for the Zyxel, which has plenty of configurations entries more. Actually the 192.168.1.0/24 network is natted, although I can't exactly understand what you mean for routed (static route on the tp-link to the .1.0 network?) – Michele Scuttari Dec 17 '17 at 19:34
  • fine: the USG 50 is a firewall. – Px2016 Dec 17 '17 at 20:12
  • Yes exactly, I forgot to mention that. Can you give me any advice for the setup? Thanks in advance – Michele Scuttari Dec 17 '17 at 20:30
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If you want all devices to be on the same subnet then bridging is the way to go.

My educated guess about how the Zyxel device does balancing across two WAN ports is that it is done in a way that relies on NAT. Once you do bridging there will no longer be any routing or NAT happening on the Zyxel device, thus you will need to find a different way to balance traffic across two interfaces.

Bundling

What you need instead is to configure the two interfaces as a bundle. And you need to configure them as such on both devices. I don't know the devices you are using so I cannot guarantee that they will support configuring two interfaces as a bundle.

Notice that in bridging mode there is not supposed to be any real difference between WAN and LAN ports anymore. So if it turns out the Zyxel only supports bundling on LAN ports you can bundle two LAN ports between the devices.

DHCP

It is quite common for a DHCP server to announce itself as gateway, but it is not a requirement. So though you could in principle have the Zyxel act as DHCP server and announce the TP-Link as gateway I wouldn't recommend it unless you have a good reason.

If the DHCP server in the TP-Link has bugs or lacks features, then you can give the DHCP server in the Zyxel a try. But I would recommend that you first try using the router acting as gateway to also act as DHCP server.

Notice that if you need many devices it is worthwhile considering a shorter prefix than a /24. Also notice that running two DHCP servers on a segment can work if done correctly. If both advertise same prefix length, same gateway, and use different non-overlapping ranges of dynamic addresses it should just work. If you want any devices with static addresses you would have to configure those on both DHCP servers. Having only a single DHCP server is easier to configure correctly, so that's the setup I would recommend unless you specifically need the redundancy provided by having two DHCP servers.

kasperd
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