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What is the impact of setting up 2 separate routers into one switch?

lextec used Ask the Experts™
I am reconfiguring our office and want to use a verizon DSL modem in bridge mode plugged into a DELL 2808 power connect switch, which in turn has two routers plugged into it (the DELL) with Static IP's and DHCP enabled on both routers.  One router will serve our private business office, and the other will serve a public area for guests to log in using wireless.  It hasn't worked so far.  The switch is not in managed mode.  Shouldn't this configuration work?  Aren't the packets forwarded thru the internet ports into the switch and out past the Bridge mode modem?  Have I caused some sort of packet storm or loop?  Thank you.
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If you have 2 DHCP servers on the same switch, you will have to vlan the switch or something, otherwise there is no way to tell which DHCP server is going to be the first to answer the DHCP request, and a pc may get an IP address from either router.
Also The DSL router will probably not forward DHCP unless you have enable "DHCP Proxy" in the configuration.

Not in bridged mode.

In bridged mode your router is literally just that. A bridge. The router on the other side of the bridge handles all routing. WHich means if you have two routers your kinda stuck....

If your dsl modem has on board routing ability you might want to try splitting your public static ip range, if you have one, into two subnets of the original external range.

If you don't have an external static range you're out of luck with the two direct;y external routers approach....

If one of the routers has a configurable DMZ port you could plug the guest router into that (since i'm guessing you have a segregated network for guests behind which doesn't touch your work LAN?), and have you gues network plug into the guest router attached to the DMZ port and have it just doing internet routing outbound. Make sure you configure the primary router which also pass through to your lan so that the dmz port cannot route back through to your private network.

Hope the options help.....
Standard approach for this kind of situation with one public IP address from the modem:

Modem <> Router #1 WAN <> Router #1 LAN is LAN #1 <> Router #2 WAN <> Router #2 LAN is LAN #2

No switch.
Routers in Gateway mode with NAT .. the normal thing.
LAN #1 is public.
LAN #2 is private.

If you like, you can add Router #3 WAN on the Router #1 LAN if you want even more isolation.

If the modem is in bridge mode and you have at least 2 public IPs available, then I'm not sure why your setup won't also work.  Just need the ISP gateway on both router WAN setups.

At least that's the shorthand version ....

Istvan KalmarHead of IT Security Division
Top Expert 2010

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