DHCP Relay on specific VLANs only?

Products cnMatrix
1

Switch: cnMatrix EX2028-P
WAPs: XV2-2

Hi

We have multiple VLANs within our environment. The EX2028 is configured as our “core” and has multiple L3 interfaces. We have enabled DHCP Relay via the EX2028-P GUI to point to our external DHCP server (Windows Server 2019). Inter VLAN routing is working and the clients are acquiring DHCP leases as expected.

Now for the problem. We want to introduce a second external DHCP server (firewall/web filter) for our Guest Wi-Fi. We have declared a new guest VLAN on our EX2028-P but without creating a L3 interface (in order to isolate this network). The new external DHCP server is tagged with the same VLAN ID as our new Guest VLAN. I believe the DHCP relay setting (enabled earlier) is relaying all DHCP requests to the original Server 2019, rather the new external DHCP that’s located within the same VLANs.

In short - Is it possible to set IP helpers/DHCP-Relay on selected VLAN interfaces, rather than applying to all VLANs (including those without a L3 interface)?

I’ve done this successfully with Cisco, Ruckus, Brocade and Meraki, but I can’t find a way of doing it here with Cambium.

Please help

Thanks

Tom

2

Hi Tom,

cnMatrix does not support association of DHCP server to selected VLAN interfaces. This feature is currently planned for later this year.

Thanks

3

On the Cambium AP, you can have a DHCP Relay per VLAN as per this screenshot.

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5

Unfortunately the switch running DHCP relay will intercept all DHCP requests coming through.
Also for the AP to relay they would need an IP in the segment which for the guest VLAN you would prefer to not have.

The issue is clearly a faulty DHCP relay implementation since the switch should not intercept DHCP requests on a layer 2 VLAN.

Being able to configure on a per VLAN basis would be good, but in most cases just not intercepting DHCP requests on layer 2 VLAN’s would be sufficient.