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This is my first time posting and I have searched for similar question to the best of my abilities yet believe none to be helpful for my cause.

My problem is, that I have a Fritz.Box 3270 as router, dns and dhcp connected to the clients on my 192.168.1.0/24 network which would be the home network with all clients being used for private purposes.

Then there is a Netgear GS724T ethernet switch with a server running several virtual machines attached to the Fritz.Box One of those virtual machines is configured as a second DHCP/DNS managing the 192.168.14.0/24 subnet, being the subnet for work-related clients.

For some reason the clients from subnet 1.0/24 keep getting DHCP responses from the 14.0/24 DHCP, thus receiving IP adresses from the wrong subnet. As far as I have found out by now, the Fritz.Box is not able to be a DHCP relay agent. So why do the DHCP requests get forwarded to the other DHCP?

And even if they do, how does the secondary DHCP manage to answer them first? It is my understanding that the router should certainly be able to answer faster since the request has to pass THROUGH it to even get to the second DHCP, which should then prevent wrong IPs being handed out to subnet 1.0/24.

Edit:

The virtual DHCP is a SLES 11 SP3 running on vSphere 5.5

Edit2: Network Topology https://i.stack.imgur.com/egllo.jpg (Not enough rep to actually post images)

Adrian
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  • A network diagram would be helpful in understanding your network topology. – joeqwerty Sep 27 '14 at 15:39
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    if you could install wireshark on one of those clients you could see the whole dhcp conversation using a simple filter (dhcp) and that would help you find out where the problem is. It really is quite simple. – natxo asenjo Sep 27 '14 at 17:23
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    correction: filter must be 'bootp', not 'dhcp' (my apologies) – natxo asenjo Sep 27 '14 at 20:13
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    Most home/small routers do not route between LAN ports. All LAN ports are instead switched Ethernet ports. – Brian Sep 27 '14 at 21:09

1 Answers1

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The problem is that you have two separate IP networks in the same broadcast domain, and therefore you don't get the behavior you want.

You need to set up separate VLANs for the different devices you have, so that there is only one DHCP server per network/broadcast domain.

Tero Kilkanen
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  • shouldn't the broadcast domains be split by the routing devices? (i.e. the Fritz.Box as well as the virtual Server? I'm guessing that the virtual DHCP is 'listening' on both NICs and not just on the one connected to the 14.0/24 side of the network. – Adrian Sep 28 '14 at 11:31
  • DHCP is set to DHCPD_INTERFACE="eth0" where eth0 is the NIC serving the 14.0/24 side of the network – Adrian Sep 28 '14 at 11:49
  • @Adrian As Brian said in another comment it may not be routing each port individually. Or to put it another way, how many IP addresses are configured on the Fritz.Box: 3 or only 2? – richardb Sep 28 '14 at 11:53
  • Not sure if I understand correctly. The Fritz.Box is only part of the 1.0/24 subnet and should only have one single internal IP, the only device part of both subnets is the virtual server, which has two NIC each of those is part of only 1 subnet with only 1 IP After checking, I'm now convinced that the Fritz.Box is in fact not routing the LAN ports but rather just acting as a switch – Adrian Sep 28 '14 at 12:49