Help with L3 DHCP Routing on M4300 series

[Agsy]

Hey guys,

I am currently trying to set up L3 DHCP routing on our Netgear M4300-series switches and I am struggeling a bit. Main problem: our core switches M4300-12x12f (as can bee seen on my attached image) are in production and I don't want to mess things up. So, here I am looking for help 😉

This is what I am trying to set up: we have a bunch of VLANs already configured on our switches and router / firewall, like VID20 for one of our client networks and VID100 for all server systems. Here we also have one of our DHCP-Servers up and running (192.168.100.3). All subnets / VLANs normally use our router / firewall as their gateway (192.168.X.254) for inter-VLAN routing.

For DHCP however we would like to use our Netgear switches to directly route that particular traffic directly to our DHCP servers.

As an example and as can bee seen on my attached image: a computer connects to a port in VID20 (access) mode and requests an IP via DHCP. We want that request directly routed to the DHCP server in VID100 (192.168.100.3). Any other traffic should be handled by our main router and firewall.

As I have never configured routing on a netgear switch and don't have a testbed at hand, I appreciate any help.

Thanks a lot in advance!

Cheers
Sascha

[DanielZhang · Accepted solution]

Hi Agsy,
Welcome to NETGEAR community!

M4300 support the function "UDP Relay" could resolve your concern that relay DHCP  service cross  different VLAN.
Please check following screenshot and user manual to deploy it.

May the solution help you

Thanks,

Daniel.

[DanielZhang · Accepted solution]

Hi Agsy,
Welcome to NETGEAR community!

M4300 support the function "UDP Relay" could resolve your concern that relay DHCP  service cross  different VLAN.
Please check following screenshot and user manual to deploy it.

May the solution help you

Thanks,

Daniel.

[Agsy]

Cheers Daniel,
thanks a lot for your detailed instructions. 

I tried to setup our Core switches as described by you, however, without great success... Here's what I did so far:
- Assigned VLAN 20 to a port for testing (Access mode)
- setup VLAN routing for VLANs 20 (client network) and 100 (Server network, where DHCP server is located)

 

---

- Configured UDP relay for VLAN 20 (tried both 192.168.100.3 (DHCP server address) and 192.168.100.254 (routing interface of router) as destination address: 

What I can see is the hit counter increasing for UDP packets in VLAN 20, as soon as I connect a device in DHCP mode. However, when I mirror that VLAN 20 port and check with Wireshark, I still see the DHCP Discover packets bradcasting to 255.255.255.255.

In case it might help troubleshooting, here is the current routing table of our core switches:
 

What am I missing?

Thanks again for your help!

Cheers
Sascha 

@DanielZhang wrote:

Hi Agsy,
Welcome to NETGEAR community!

M4300 support the function "UDP Relay" could resolve your concern that relay DHCP  service cross  different VLAN.
Please check following screenshot and user manual to deploy it.

May the solution help you

Thanks,

Daniel.

---

[DanielZhang]

Hi Agsy,

Could you please check the Server IP is available(pingable) on network?

M4300 will try to get ARP(IP/MAC) for server IP before relay packets.(Usually use a ping for it)

Because M4300 need to know the actual IP address and MAC address of server

So if your server IP is behind firewall or not pingable.

We need to add static ARP for it that M4300 will forward correctly with DHCP L3 relay.

Let me know if you have further concern.

Thanks,

Daniel.

[Agsy]

Hi Daniel,

sorry for my late response to your last suggestion. We finally got everything up and running, thanks to your valuable input. 

In the end it was just a missing modification to the Windows routing table on our Windows-DHCP server. UDP relaying was working all the time, but the return packets were just mis-routed 😉

I marked your first reply as a solution for my problem.

Cheers

Sascha