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Old 11-02-2012, 01:46 PM   #1
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Default 2 ISPs, 2 Routers, 1 network

I have an interesting situation and I was wondering if anyone here could help me understand my network.

We have 30 computers in our office, and given our geographic location, we have limited options when it comes to internet connections. 1 DSL line proved to be insufficient, and boosting speed wasn't an option, so we ordered a 2nd line.

We have 2 routers now, 1 for each DSL line. We have 1 shared printer, and 1 shared file server. Here's how the network is currently set up.

..................................File Server
Internet --- Modem --- Router A --- Hub --- Printer
.................................................. .|
...............Internet --- Modem --- Router B

Router A is, Router B is Right now, DHCP is enabled on both Routers, with non-overlapping scopes. Both routers have assigned the same IP to the printer. If I connect wirelessly to router A, I can get to router B's setup page by entering it's internal IP, even though I have an IP that's in the scope of router A. I'm guessing it's going through the hub (yes, hub, not switch).

What I'm primarily concerned with is evenly distributing the hosts between the modems so that neither one gets bogged down and slow. The issue is that it seems to be a crapshoot as to which router is actually the gateway. For example, if I open the wireless connections dialog in Windows and pick Router B's wireless connection it's possible that I'm merely using Router B as an access point, and I end up getting Router A as my DHCP server/gateway/internet. So, it will say I'm connected to Router B, but clearly I have an IP that's within the scope Router A. Either way, I can still print and access the file server.

If I want to make half the computers use one DSL line, and half of the computers use the other, do I have to manually configure each computer or is there some way around this? Aside from that, are there problems that I'm not foreseeing here? i.e. IP conflicts, hardware freakouts, etc.

Last edited by derekalv911; 11-02-2012 at 01:49 PM.
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Old 11-02-2012, 01:51 PM   #2
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Best way to do this is to use a gateway redundancy protocol such as VRRP or GLBP.

GLBP would be the preferred method, as it's easier to do load-balancing.

Basically, the way it works is that you have two routers which are configured to answer to the same IP address. One router is the "master" which determines the load balancing configuration. Each ARP request for that shared IP address gets the MAC address for one of the two routers. The hosts are distributed in that way. Then you'd have your DC handle DHCP.

Obviously, this requires a real router and not a SOHO device. Cisco 891 or Juniper SRX100 would be perfect for it.
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Old 11-02-2012, 02:34 PM   #3
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ok, thanks.
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Old 11-02-2012, 07:54 PM   #4
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Your LAN design seems a bit confusing.

Essentially, it seems you have two separate subnets/broadcast domains, which is not really required in this case.

I would suggest placing everything in one subnet, preferably a /24 subnet mask. With 30 nodes, your broadcast domain will not be so large, and it will simplify things a lot.
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Old 11-02-2012, 11:34 PM   #5
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You can not have two independent connections on one Network.

You have to configure two independent Networks (one for each Modem & Router) using different subnet for each.

You need one computer (on any of the networks) with two Network cards, then Bridge the two Networks through the computer with the two cards.


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Old 11-04-2012, 11:33 AM   #6
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Originally Posted by JackMDS View Post
You can not have two independent connections on one Network.
Yes and no.

The simplest scenario is that you do have one local subnet (LAN) and a default route. Anything destined to the LAN is simply thrown out of the interface. Everything not destined to the LAN will be sent to the device "GW" in LAN that is mentioned in the default route. The GW is assumed to be able to route the packets towards their real destination.

It is however possible to have multiple devices on the LAN that can route. The question is how the sending hosts can decide which route to use. That is routing configuration. It is simpler if one machine acts as router that is between the uplink modems and the local subnet.

If modem/routers can do DHCP, only one may do so to common subnet, or else they do provide conflicting information.

Originally Posted by JackMDS View Post
You need one computer (on any of the networks) with two Network cards, then Bridge the two Networks through the computer with the two cards.
A bridge is a software switch. When something comes from one port, it is transmitted to other ports (but bit less after "learning"). Therefore, the same one local subnet will be on both sides of the bridge. A router connects two different subnets. (Additionally, a host can be just "multi-homed"; having interfaces to multiple subnets but neither bridging nor routing between them.)
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Old 11-05-2012, 01:00 AM   #7
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Bond the DSL connections with MLPPP, then you'll have one router.

Check with your ISP if they support this? If they don't change to one that does.
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Old 11-05-2012, 09:20 AM   #8
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Save yourself the trouble and bond T1 lines. It will be more expensive but provide you with better service and ability to scale your bandwidth up and down. Otherwise, do you have an option for business class cable?
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Old 10-21-2015, 03:19 AM   #9
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Search network specialist:
2 Networks (or 3…) together:2 houses to share files and camera directly, and the second level independent except internet access by Wifi:
my home (ISP = Voo and modem /router NetGear)
my society with 2 levels: level 1 society and second level (student kots) (ISP= Belgacom and BBox2) and 1 router Dlink DIR-868L. Students connected by WIFI on BBOX. Society computers connected on DIR-868L via Wifi or cable and switch.
If I connect the 2 houses with a bridge (2 Dlink DWL-2100) , the slowest part of the network, some computer of my house are slow on internet because gateway send it on the ISP of the other house.
Can you say me how force computer to connect the ISP inside his house, speedest?

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