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My retail shop uses Comcast Cable (calling it CC going forward) as the ISP. I just added Verizon 5G Business internet (calling it VZ going forward) as the backup for the times when CC is down (which has been more often than I like).

The LAN IP for the CC router is 10.1.10.1
The LAN IP for the VZ router is 192.168.0.1

The problem is that every time when a device connects to the VZ network, it will get assigned, by the VZ router, the default gateway of 192.168.0.1 and an IP in the 192.168.0.x range, which is fine, but when I'd connect back to the CC network, somehow the 192.168.0.1 default gateway and the 192.168.0.x IP get stuck in the device. Now, this device cannot be seen in the CC network nor can it go to the internet. I have to manually renew the IP on the device so that it will go back to 10.1.10.1 default gateway and a 10.1.10.x IP.

I am thinking:
I will change the VZ router’s LAN IP to be the same as the CC router's 10.1.10.1
I will then set the VZ router’s DHCP range to be the same range as that in the CC router.
We would NOT connect to the VZ network unless CC is down. This should avoid the conflict of having two routers with the same LAN IP.

Will this solve the problem?

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2 Answers 2

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Trying to let your clients decide where to go when is the hard way. Host configurations are off topic here, so we can't much comment on that.

The easier way is to use a router that decides which uplink to use. Most business-grade routers can use more than one WAN link. You can run the links in active/passive mode, or distribute load between them when both links are up (by e.g. source, destination or protocol) and fail over to the other side when one drops.

Very often a link doesn't fail on the physical layer (ie. link loss) but it just fails to provide connectivity. Your router should be able to monitor connectivity to a known-good service (sometimes called SLA with ping) and use that to decide where to route.

I will change the VZ router’s LAN IP to be the same as the CC router's 10.1.10.1 I will then set the VZ router’s DHCP range to be the same range as that in the CC router. We would NOT connect to the VZ network unless CC is down. This should avoid the conflict of having two routers with the same LAN IP.

You should only have one DHCP service running, not conflicting ones handing out addresses from the same scope. Even if you use a single IP subnet, your clients still have to decide which default gateway to use. Unless you implement a router redundancy protocol with a virtual IP (VRRP or HSRP) you'd need to move the LAN IP from one router to the other or reconfigure all clients. That seems troublesome.

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  • But I am connecting to only one network at a time (normally we will NOT be connecting to the VZ network.)
    – RA2024
    Commented Mar 31 at 18:20
  • Then it's the active/passive scenario above. Remember, it should be you who's defining your network, not some ISP router.
    – Zac67 ♦
    Commented Mar 31 at 18:51
  • For reasons that I don't want to get into now, I try to avoid using a dual-WAN router (even though it may be the most proper solution in the professional world.) This is more of a work around for us for now. I will leave the VZ gateway power off when the CC network is in use,and, only turn the VZ gateway on when CC is down. In light of this, will my setup work?
    – RA2024
    Commented Apr 1 at 0:55
  • It will not work well and will always require manual intervention to make things work. The dual WAN router is the correct solution but if you don't want to use it, you could try to have the routers from the ISPs provide a very short duration DHCP lease time so client computers will renew their DHCP lease more aggressively. Or once you move the backup connection, simply reboot every device on the network.
    – FrameHowitzer
    Commented Apr 1 at 14:53
  • With a Dual-WAN router, the IP addresses and default gateway ip on the devices will get renewed immediately and automatically when the WAN is switched from one to the other (in a Fail-Over configuration)?
    – RA2024
    Commented Apr 1 at 15:00
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Without designing the solution, you might consider this approach: Since you are already willing to turn off one of the routers then you might just use that approach altogether. Then you should be able to safely give each router the same LAN IP since only one will be present at any time.
If there are assigned IP addresses per MAC address in the router then they would be made the same when you set up the routers. You may find that ARP caches need to be refreshed; such as in switches. One way in small organizations that has worked well is to have a master power switch that will reboot all the network equipment - this can also be helpful if you tell clients: reboot the network before you call me! Of course, as things get bigger and more complex, this may be harder to accomplish. From personal experience, I wouldn't suggest that refreshing the ARP caches is necessarily needed. I had one network that could switch ISPs in one direction with no problem but would have to be reset when returning.

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  • While that would likely work, I wouldn't call it good practice. When even a simple (off-topic here though) WRT router can provide dual-WAN connectivity there's very little reason for makeshift workarounds like that.
    – Zac67 ♦
    Commented Apr 2 at 17:31
  • Either way, a major objective is to get this system into a single LAN subnet - which I may not have emphasized enough. Having ready replacement equipment pre-configured and plug--n or switch-in ready is good practice in small systems and, maybe, in larger systems in a well-considered failover approach. I don't disagree with the suggestion of using a dual-WAN router in this case, simply that RA2024 rejected it. Given that constraint, something like this appears to be what's needed. @RA2024: Yes, that will work given the ARP cache situation is handled (if it exists).
    – fred
    Commented Apr 2 at 18:12
  • You might consider the dual-wan router as having two routers for ISP connections that are internally connected to ONE LAN interface with normal router internal LAN settings available. There's not much to it - just don't repeat the ISP subnet ranges on the LAN side. Normally you willl have public IP addresses on each of the WAN connections.
    – fred
    Commented Apr 2 at 21:38

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