Will there be issues with running 2 DHCP servers on one network?

[AlejandroL]

Ok. Just trying to figure out what the draw backs or issues would be in setting up 2 DHCP servers on the same network.
First, I have the setup explained in the thread http://www.tomshardware.com/answers/id-3225182/vpn-cascaded-router-connect-anymore.html which is working fine as is now, but the response I just got back from my ISP regarding that problem makes me thing that I probably want to make use of their solution, but I'm just not sure regarding the part of having 2 different DHCP servers running on the same network at the same time.

According to them, they provide the ability to connect either 2 devices to the internet in layer 3 service thru the 1st and 2nd port of the zhone 2520 ONT fiber optic modem/router they provide, or 1 device on the 3rd port of the ONT which they deliver in layer 2 (and port 4 is used to connect the TVs).
They instruct me to do the following:
1. Connect my tplink 750ac router's WAN port to the 1st port of the zhone 2520 ONT we provide for local internet. This port is a DMZ port. It assigns a fixed IP to that port of 192.168.15.150
2. Connect my second tplink 750ac (now a d-link dir 855) router's WAN port to the second port of the ONT. This port has VPN, L2TP, PPTP, IPSec passthru enabled. It assigns a fixed IP to that port of 192.168.15.151 (Office VPN thru L2TP)
3. Configure router 1's WAN ip to be dynamic
4. Configure router 2's WAN ip to be the VPN using either L2TP or PPTP.
5. On router 1's LAN set:
- ip 192.168.1.1
- mask 255.255.255.0
- gateway 192.168.1.1 and any valid dns servers.
- enable DHCP in the range of 192.168.1.50 thru 127, mask 255.255.255.0 gateway 192.168.1.1 and any valid dns servers.
- name WiFi something unique like "WiFi Local"
6. On router 2's LAN set:
- ip 192.168.1.128
- mask 255.255.255.0
- gateway 192.168.1.128 and any valid dns servers.
- enable DHCP in the range of 192.168.1.177 thru 254, mask 255.255.255.0 gateway 192.168.1.128 and any valid dns servers.
- name WiFi something unique like "WiFi Office VPN"
7. Connect router 1 to router 2 thru LAN ports with ethernet cable.

According to them, with this setup anything that gets physically connected to router 1, or that uses the "WiFi Local" connection will get assigned an IP that will go thru the "local" network, and that anything that is physically connected to router 2, or that uses the "WiFi Office VPN" will go thru the L2TP connection to the Office's VPN. They also recommend that fixed devices like the printers and the NAS servers be assigned a manual IP from 192.168.1.2 to 49 if they are going to go out thru gateway 1 (192.168.1.1) or IPs from192.168.129 thru 176 if they are going to go out thru the second router's gateway 192.168.1.128.

This makes a lot of sense, and I would venture to say that it should work, but I've also read that having more than 1 DHCP server running on a single network is a BIG NO, NO!
What would be the draw back to connecting things in this manner?
Am I going to have issues with both routers trying to give out IPs?
Will this slow down the network?
Or something else you guys might foresee as a problem?

Thanks in advanced.
Alex

BTW, and JFYI the reasons I am considering switching to this manner is that I would be able to access all my resources no matter to which gateways I was running thru (printers, NAS, etc.), and according to the email response from my ISP, I get double my speed for free for the time being since they have it set up right now to give both port 1 and port 2 of the ONT the full bandwidth I am paying for independently..

 

[jsmithepa]

You can only run more than 1 DHCP server if said DHCP are configured as Master-Slave or Master-Backup such as done on a Microsoft Enterprise Network.

 

[AlejandroL]

You can only run more than 1 DHCP server if said DHCP are configured as Master-Slave or Master-Backup such as done on a Microsoft Enterprise Network.

Why if they are not using the same IP pool to give out? This is not a split scope dhcp setup, nor a failover setup. Each router handles a part of the ip pool. Depending on which one actually assigns the ip is the gateway that gets assigned to that device.
What are the drawbacks?
Or what issues will arise?
There's no more than 20 total devices that will ever be connected to the network, and most will be fixed ips with their default gateway set. So what are the problems with this?
Isn't the router that the device connects to smart enough to handle the request? Or are both routers going to send out assignments? If so, isn't the device smart enough to pick one to use?
I can see that it might be a problem with wired devices that might get conflicting assignments, but wont the wireless devices get the correct ip and gateways from the router they actually connect wirelessly to?
This is what I am trying to find out, so please explain why not and what the issues would be please.

 

[maxwellmelon]

because the questions your asking is not really a simple question

http://serverfault.com/questions/368512/can-i-have-multiple-dhcp-servers-on-one-network

yes you can run multi-DHCP severs on a network......if everything is configured right and the needs of the network, such as every device on the network need to talk to other device on the network or is everything just need to talk to a central server/internet, the devices of the network allow the configuration needed to setup a complex network. There's at reason people get trained and certified in networking. Off hand nothing in your OP stands out as looking incorrect to me. Just asking though is the person a professional networker? if so I would go with there advice.

 

[bill001g]

The only flaw in your design is the assumption that you will get a ip from the device you have physically hooked your pc to. Since you have connected the lan together both routers will see the request for ip. They will then both respond. If there was a lot of latency between the routers then the local one would mostly be preferred. When there is almost no latency you will get pretty much random results.

This is the problem with multiple dhcp servers there really is no way to predict which will be selected.

 

[maxwellmelon]

The only flaw in your design is the assumption that you will get a ip from the device you have physically hooked your pc to. Since you have connected the lan together both routers will see the request for ip. They will then both respond. If there was a lot of latency between the routers then the local one would mostly be preferred. When there is almost no latency you will get pretty much random results.

This is the problem with multiple dhcp servers there really is no way to predict which will be selected.

True but the network would have ip address safely for up to 77 devices if he configures the ip address ranges as set above. but as you state wouldn't be able to 100% force which DCHP server to use as the first one to respond will be what the requesting device will use.

 

[bill001g]

The only flaw in your design is the assumption that you will get a ip from the device you have physically hooked your pc to. Since you have connected the lan together both routers will see the request for ip. They will then both respond. If there was a lot of latency between the routers then the local one would mostly be preferred. When there is almost no latency you will get pretty much random results.

This is the problem with multiple dhcp servers there really is no way to predict which will be selected.

True but the network would have ip address safely for up to 77 devices if he configures the ip address ranges as set above. but as you state wouldn't be able to 100% force which DCHP server to use as the first one to respond will be what the requesting device will use.

Agreed it is much less a issue if both dhcp servers were using the same gateway. In his case he will in effect be assigning his machines to a random internet connection in addition to assigning a ip out of a random pool. Since he calls the second one "office vpn" makes me think this is his connection to work. I would assume that random assignment to this network would not be the optimum thing to be doing.

 

[BuddhaSkoota]

If I understand correctly, this is the OP's proposed configuration:

If correct, DHCP services are isolated, so requests by devices will not be seen by both routers (i.e. will not traverse router WAN ports).

It's 2 separate LAN segments.

If both of the ONT ports are working in the manner that the ISP is indicating, the only issue I see is double NAT.

 

[AlejandroL]

Yes, that is correct, but the LAN of router 1 is connected to the LAN of router 2 thru an ethernet cable.
Like I said, I see the issues with anything wired, but that can be fixed since I can manually add the ip and gateway for everything connected physically.
I guess I asked the wrong question, in this case the question should have probably been, using the above setup, if I connect a wireless device to, for example the "WiFi Office VPN" wireless network, the request should be answered by router 2 and give me the gateway to the office VPN? Or eventhough I am connecting to that wireless network, there is still a possibility of getting a response from the first router's DHCP server?
In this same example, I want to connect my laptop to the office, and while using that, be able to print to the printer at home, and be able to save any relevant files to my home NAS for example... In this case both the printer and the NAS would have static ips with the default gateway set to 192.168.1.1

 

[bill001g]

Yes it can still get a IP from router1. The wireless chip is in effect connected to the lan switch as if it were a AP. The router chip also connected to this same internal switch. Although the router chip is technical a little closer you can not depend on that alone to guarantee which dhcp is selected. If the router chip happen to be a little busy when the request came in the other router may respond faster.

 

[BuddhaSkoota]

Yes, that is correct, but the LAN of router 1 is connected to the LAN of router 2 thru an ethernet cable.
Like I said, I see the issues with anything wired, but that can be fixed since I can manually add the ip and gateway for everything connected physically.
I guess I asked the wrong question, in this case the question should have probably been, using the above setup, if I connect a wireless device to, for example the "WiFi Office VPN" wireless network, the request should be answered by router 2 and give me the gateway to the office VPN? Or eventhough I am connecting to that wireless network, there is still a possibility of getting a response from the first router's DHCP server?
In this same example, I want to connect my laptop to the office, and while using that, be able to print to the printer at home, and be able to save any relevant files to my home NAS for example... In this case both the printer and the NAS would have static ips with the default gateway set to 192.168.1.1

Sorry, I did inadvertently overlook the router LAN-to-LAN connection, so the previous replies about two DHCP servers would apply. Unfortunately you couldn't guarantee a response from only the intended router.

I don't think that the LAN-to-LAN connection is the best idea, and would recommend ditching it. When connected via VPN on the office computer, you might be better served using a web print application to print to a local computer. (e.g. Google Cloud Print). Having work documents on a home NAS is probably not a great idea either, but using FTP or cloud services on the NAS (with a specific IP restriction to your virtual IP address) might be a possible solution.

 

[AlejandroL]

Yes it can still get a IP from router1. The wireless chip is in effect connected to the lan switch as if it were a AP. The router chip also connected to this same internal switch. Although the router chip is technical a little closer you can not depend on that alone to guarantee which dhcp is selected. If the router chip happen to be a little busy when the request came in the other router may respond faster.

Great, that's what I wanted to know.
In reality the setup should still work just leaving one DHCP server running, and if I need to connect anything to the Office VPN just manually configure the device by hand to use router 2's gateway.
This is probably even better since I can then just give the WiFi network a single name on both routers and use it to increase the range of it using it as an AP.
Thank you all for your help.

 

[feitosa.arfneto]

It is an old thread, but I as wondering if you could not just use the firewalls on the 2 routers to block the DHCP traffic from crossing the LAN-LAN connection between the 2 routers. As the OP said, it is not a failover or split scope setup...

The routers have fixed addresses and blocking traffic on ports 6[78] UDP would kill any race condition between the 2 servers. Is it possible?

 

[bill001g]

Without rereading all this old stuff I strongly suspect these are consumer grade routers. The firewall rules only apply to traffic going wan-lan. Even in commercial equipment it is not real common to be able to filter on layer 2 ports.