i am contemplating the idea of switching our current FG 80D to a 90D and implement SD-WAN using our current 100Mbit fiber circuit (main) and the 4G LTE (cradlepoint) for internet backup.

Our current ATT router for the main circuit is doing an internal natting for VoIP and using one of the public IP in the block assigned.

No this is the first time i set up SD-WAN and i'am trying to answer a few questions related to it:

  1. If we use the second circuit - cradlepoint (the SIM has a static public IP) with IP Passthrough, internet would of course be up, but what happens to VoIP? Assuming that just the fiber is down, but the ATT Router is working ok, can the VoIP still be up if the active circuit is now the cradlepoint?
  2. Could i work with subinterfaces to bring a second VPN tunnel up in case we are accessign the internet with the backup circuit?

Thanks, DT

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1 Answer 1


The public IP is nailed to your AT&T router which will be down when it's down. BGP would be required to tell the Internet that your public IP is now accessible via another link.

I don't know of your VoIP config/topology so I can't say much more without more info. You might be better served configuring a redundant VPN as a foundation. From there, your internal IP traffic (including VoIP packets) would have redundant paths. However, if your VoIP system needs to be reachable via the Internet, there are other steps you might be able to take depending on the VoIP protocol, such as having secondary/failover addresses for whatever VoIP service it is.

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