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SBS2011 box behind router behind modem setup

Your help in setting up an existing sbs server  is appreciated.

The background:
We have an sbs server that used to connect directly to either our cable modem or our dsl depending on which had an outage.

Both the cable modem and dsl modem had the same IP address ( with deco turned off and appropriate ports forwarded to the sbs server which has up address (

One day the cable modem hiccuped and dhcp turned itself on which caused havoc with the sbs server so now the server is only on the dsl modem.   We can't figure out where the dhcp is on the cable because all settings are the same.  

We still want to use both when there is an outage on one, like now that our dsl is down.  

How can we set up a router between the sbs server and either modem?

Do we use a router, turn off dhcp, and have the cable modem port forward to the ip address of the router?   Then have the router port forward to the sbs server?

If so what IP address do we assign to the router?  Do we turn dhcp back on on the cable modem and dsl modem and still port forward to the router?

I'm stumped and your help is appreciated.
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Michael Ortega

8/22/2022 - Mon
Michael Ortega

Maybe a stupid question, but if you have to failover from one service to the other, undoubtedly the public facing IP is going to be different. Are you actually manually (or using some dynamic DNS service, e.g. dyndns, dnsmadeeasy, etc.) changing public DNS records to forward traffic through the connection you failed over to?

At any rate, my recommendation is using a link balancer or link aggregator. Alternatively, you could put a router or firewall in between the cable/dsl service(s) and the LAN where the SBS resides. With a Router or Firewall you could connect both your WAN Services and setup some kind of IP SLA to determine when to failover to one circuit of the other. The modems/routers for both service providers should be bridged to allow your firewall/router to be public facing.

Link Balancer option: https://www.barracuda.com/products/linkbalancer/models
 - The 230 would be plenty adequate.

For firewalls, their are tons of options. I personally like Cisco products, e.g. Cisco ASA (5506-X would be adequate for your needs).

Eric C

I could be wrong, but since both of your modems are doing NAT, it sounds like they are both routers. So I am not sure if inserting another router in between (and doing dual NAT?) is beneficial.

You're trying to find out where (on the cable modem) DHCP is enabled? are you able to log into its web interface? Depending on the device, maybe there's an 'Advanced' mode and only there will you see the DHCP option.

If you have a small(ish) network, consider turning DHCP off and assigning manual IPs to all of your devices (computers, printers, etc). (People will probably snarl at this since in a well-designed network, this is nothing but an added complexity).

Either way I'd definitely disable DHCP on both internet devices. Also, your SBS prefers (wants) to be the DNS for your Microsoft domain.

How were your computers getting out to the internet, if your SBS was connected directly to the modem?
Eric C

Michael Ortega, can you tell us more about this Barracuda Link Balancer? Let's say my primary ISP gave me a few public IPs, and those public IPs are mapping to servers in my DMZ. (Email, FTP, Web, etc).

Now let's say my primary Internet connection goes out and my backup internet kicks in. Now what? Wouldn't all my IP services be down as well?

How do these devices address work, in this scenario?
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William Peck
Michael Ortega

The Link Balancer acts as a single device/gateway to trusted resources inside. The rules are setup for forwarding just like any router/firewall. There are ACLs to allow traffic inbound. You would need to bridge the carriers' routers/modems first though.


Thanks for your suggestion.

So would this be a correct synopsis?

In my case a dual wan router installed and have both the dsl modem and cable modem bridge mode so that the dual wan router is assigned the IP address by Comcast and ATT respectively.  And then configure port forwarding on each wan from within the dual wan router?

In effect I'm combining both modems into one box and controlling the box like I used to control each modem individually?

Is that correct?

In answer to the other questions:

We do use dynamic dns updating at dyn.com to make sure we receive our email etc.  

I've tried everything on the cable modem and drilled down every menu to find the dhcp server but no luck.  I think the cable modem went crazy with a glitch.


A follow up to my comment.  If the above I wrote is correct, and my sbs server has a static ip of and is configured to use as the gateway, would the dual wan box have IP address of

I'm trying to confirm that the dual wan router would then take care of balancing the outbound traffic.  

If each wan connection would have a different static IP address then that couldn't work well.  But I imagine it's the latter.

The barracuda looks good but may be overkill for our 4 person office. CISCO SYSTEMS Gigabit Dual WAN VPN 16 Port Router (RV325K9NA) Looks like it may work AND replace our switch to save space.
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