Lancom Router - Zyxel GS1920 - FritzBox (as Router) - DHCP not working

Kallewirsch · January 2023

Hi,

I have a relative complex setup.

- my LANCOM 1902VA works as Internet Interface and DHCP Server

- my ZYXEL GS1920 works in this setup as Access Switch

- attached to the GS1920 is a FritzBox configured as Router

The connection to the FritzBox works. The FB even grabs its IP-Address via DHCP (setup in the LANCOM as BOOTP entry via MAC).

As you know, the FritzBox serves as WLAN Access Point and within this configuration it serves its own IP-Network.

Normally, the FritzBox also works as DHCP-Server for its WLAN IP-Network. But I want to do the DHCP for this WLAN Network by the central DHCP Server (Lancom).

I simply have switched off the DHCP-Server Option of the FritzBox WLAN IP-Network. I assumed the DHCP-Request for WLAN Clients to be distributed via the Zyxel to the Lancom and get the answer. But it does not work.

Help appreciated!

Regards

Kallewirsch

Zyxel_Melen · January 2023

Hi @Kallewirsch,

Since the FritzBox grabs its IP Address via DHCP, your switch works properly.
You could mirror the FritzBox connecting port or the LANCOM connecting port.
This allows you to check if the FritzBox did not send the DHCP discovery packets out or the LANCOM did not send the DHCP offer packets.

Kallewirsch · January 2023

Hi Melen,

thanks for the answer.

I am sure there is somewhere a description how to mirror a port on a zyxel and trace the relevant information.

Perhaps you could send me a linke to this description if somewhere available within the KB of Zyxel. If not, I will find somewhere.

Thanks for help and have a nice day!

Regards

Karl-Heinz

Zyxel_Melen · January 2023

Hi @Kallewirsch,

You could follow the below steps to setup port mirroring:
1. Go to Advanced Application > Mirroring.
2. Setup Monitor port as 1(I assume the PC port is port 1) and enable mirroring.
3. Port 9 enable mirrored and set as both direction (I assume the FB/LANCOM port is port 9).

Image: https://us.v-cdn.net/6029482/uploads/editor/kq/xift27i9izlh.png

4. Open the Wireshark to capture the packet.

Kallewirsch · January 2023

Zyxel_Melen said:

Hi @Kallewirsch,

Since the FritzBox grabs its IP Address via DHCP, your switch works properly....

Hi Melen,

sorry to come back on your first answer!

The FritzBox is configured as Router. So it has a WAN-Port (Part of the Network of the LANCOM) and serves an own IP Network.

Example:

Lancom has 192.168.114.1/24

FritzBox WAN-Port has 192.168.114.2/24 via DHCP.

For this DHCP-Request it was quite clear the DHCP-Request would work. The Zyxel Switch inbetween is for this transparent.

The FritzBox offers its own IP-Network, say 192.168.188.1/24 for WLAN.

If DHCP-Server for this WLAN Network is disabled, all DHCP-Request need to be transferred to the LANCOM.

I am not a Network Expert but I have in my memory that DHCP-Requests are issued via Broadcasts and Broadcasts cannot be routed.

So the DHCP-Request from the WLAN (192.168.188.1) through the WAN-Port of the FritzBox (192.168.114.2) needs some routing - if I am not wrong. But I might be wrong!!!!

I have heared about in such a case, that there needs to be a DHCP-Helper on the attached Switch????

Not sure if this is all correct. But it is fact, that the DHCP-Requests are not issued. I have setup the Port Mirroring on the Switch and connected my PC to the Mirror and have run Wireshark on this to trace the traffic.

I have seen the DHCP-Request from the WAN Port of the FritzBox to the LANCOM with a DHCP-Offer - but this was still clear to work.

I have not seen any other DHCP-Requests e.g. of MAC-Addresses of WLAN Devices (Smartphone ....) trying to connect.

There is probably no solution for this. But I still think this should be possible to work.

Any idea appreciated!

Regards

Karl-Heinz

Kallewirsch · January 2023

Hi,

based on several information I grabed from the net I think there is no solution for this setup.

Some DHCP-Helper (or DHCP-Relay) needs to be setup on the FritzBox, which is not possible since this is a closed system.

Issue closed!

Zyxel_Melen · January 2023

Hi @Kallewirsch,

Sorry, I thought you changed the FritzBox to bridge mode.
Since I'm not familiar with FritzBox, does its WLAN and LAN are in the same LAN?
If yes, you could try changing your topology as below:

Image: https://us.v-cdn.net/6029482/uploads/editor/ai/d9kgaac4tbk0.png

This topology allows the WLAN clients can send the broadcast to LANCOM via the LAN interface.

Here are the steps to set up the VLAN on GS1920.
1. Please go to Advanced Application > VLAN > VLAN configuration > static VLAN setup to create VLAN 2 for WLAN. I use port 1&2 as VLAN members.

Image: https://us.v-cdn.net/6029482/uploads/editor/ya/6u8tfb9sbbbp.png

2. Click VLAN 1 below to modify VLAN 1 setting.

Image: https://us.v-cdn.net/6029482/uploads/editor/av/oi9nobmr6cd5.png

Since port 1&2 only used for VLAN 2, they should be changed to "Normal".

Image: https://us.v-cdn.net/6029482/uploads/editor/rd/s2ae7d4vy2fq.png

3. Please go to Advanced Application > VLAN > VLAN configuration > VLAN Port Setting to configure the PVID of port 1&2.

Image: https://us.v-cdn.net/6029482/uploads/editor/lw/m9nsf48n4kqa.png

4. Connect the devices and verify.

Hope it helps.

Zyxel_Melen · January 2023

Hi @Kallewirsch,

Just want to recap your main propose is to make FritzBox WLAN client get DHCP IP from LANCOM, right?
And your topology is "Lancom Router - Zyxel GS1920 - FritzBox - WLAN - Wifi client"?

If both are yes, then you could consider changing the FritzBox to bridge mode. Then the WLAN traffic (include the DHCP packets) will be forwarded to LANCCOM.
When the FritzBox uses router mode, its WAN and LAN are isolated. So, the WLAN DHCP packet cannot be forwarded to LANCOM.

Kallewirsch · January 2023

Zyxel_Melen said:

Hi @Kallewirsch,

... bridge mode. ...

Hi Melen,

thanks for reply. I am not sure what you mean with "bridge mode". I know to setup the FritzBox as "IP-Client". So the FB is part of the network of the Lancom and the Switch is transparent.

But in "IP-Client" Mode there are some Features like "Guest-WLAN" not available. They only work in Router Mode. This, and some other Features, I would like to use but leave the control of the DHCP at the LANCOM.

Comment to your setup of your last proposal: The "LAN" of the FritzBox is in Fact a "WLAN", so no cable to connect to the switch. So the Network (e.g. 192.168.188.1/24) the FB serves (including DHCP) is for WLAN Clients.

Leave it for now - the setup I want is simply not possible. Perhaps FB OS will get some new features in the future. We will see. Thanks for help!

Lancom Router - Zyxel GS1920 - FritzBox (as Router) - DHCP not working

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