Hello,

I've got a Vigor 2927 that is connected via port P4 to a switch via a VLAN trunk port (with VID10, VID20 and VID30 configured to LAN1, LAN2 and LAN3 on the Vigor).
My IP subnets are 10.10.10.0/24, 10.10.20.0/24 and 10.10.30.0/24 for LAN1, LAN2 and LAN3. On the DrayTek I also have those LANs configured on ports P1, P2 and P3.
My DHCP server (10.10.10.10) is in LAN1 (VID10) and I have configured in DHCP Relay in the settings for LAN3. On the DHCP server the scope for the LAN3 subnet is added.

However the DHCP server is not giving out any IP addresses to the devices in LAN3 when the Vigor is connected over the VLAN trunk port.

When I connect the Vigor to the switch using P3 and using an untagged port in VID30 on the switch it is working okay. (Also okay when directly connecting a device to P3).
When using the DHCP server in the Vigor itself it all works fine and all VLANs (connected over the P4 trunk port) get their correct IP addresses.

What am I doing wrong? Or is not possible to use DHCP Relay over a trunk port?
I hope I described my setup clear enough; any advice?
Thanks
Duncan

I have a similar issue where DHCP relay is not working properly on the Vigor 2927. I have VLANs setup on LAN2, 3 and 4 with DHCP relay enabled and no devices are getting IPs from the corresponding subnets setup on the Windows DHCP server. LAN1 isn’t an issue as this is on the same subnet as the DHCP server and devices don’t seem to have a problem obtaining an IP from the server. For the other LANs/VLANs, when running a wireshark capture, you can see the packets hit the DHCP server and it sends a response, however that’s where it ends and client isn’t receiving the response.

Ports on the 2927 have been ticked for the relevant VLANs (for trunk port access) and inter-LAN routing is enabled for all. Small Cisco switch is setup with a trunk port back to the 2927 and other ports on the switch are setup and tagged with a specific VLAN. Also my master Ruckus Unleashed Wireless AP connects to the 2927 via ethernet and SSIDs are setup for each VLAN.

Interestingly the Vigor 2925 I was using before I upgraded to the 2927, works with DHCP relay using firmware version 3.8.9.8 with the same LAN/VLAN setup and Windows DHCP server. However after I upgraded the firmware to version 3.9.0 on the 2925 this breaks DHCP relay. Something definitely going on here.

I also tried downgrading to all the previous firmware versions on the 2927 and all versions had the same DHCP relay issue.

Starting to go a bit mad by this point, so to test further I setup another Cisco switch (with layer 3 capabilities) with interfaces on each VLAN and an IP helper on each, back to the Windows DHCP server and all works fine as you’d expect. So nothing wrong with my DHCP server setup as far as I can see. Just the router not routing!

Using the same principal setup as the Cisco switch. I wiped the config of my 2925 and reverted back to firmware 3.8.9.8, setup and replicated all the LAN/VLAN config including DHCP relay that’s on the 2927, making sure to set the IPs on each LAN unique to the ones on the 2927 (e.g. VLAN20: LAN2-2927 - 10.0.20.1, LAN2-2925 – 10.0.20.2). Plugged this into the network and clients can get an IP from the Windows DHCP server. Unplug it and no clients get an IP.

Currently having both the 2927 and 2925 routers (or capable switch) plugged into the network is the only workaround I can use to get my LANs/VLANs working with the DHCP server.

I reported this issue to UK DrayTek support a couple of weeks ago, on 22nd April. They have promised to look into it. However I’ve not had confirmation they have tested or replicated this issue yet. Hopefully something is found and fixed ASAP, as it looks like this issue could be affecting multiple devices and firmware versions.

Quite how this has slipped the net I don’t know, unless its my own setup that isnt correct and the older routers and firmware have been kind to me for the last few years!

Thanks for this extensive reply.
It sounds pretty much as exactly the same issue that I experience.
My workaround is extra cables between the 2927 and the switch as it does work when not using the trunk port.
I’ll guess it is just a bug that must be fixed; I will see how I can report that.
Duncan