Help please with connecting two routers together

DrD787

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Right at th outset let me say I have read the sticky above but I don't think it covers what I am trying to do.

I have a network which uses a TP-Link 3420 3G router. I want to add a second router to give me fallback to an alternative ISP (I am installing this in Africa and the ISP service is patchy to say the least). The TP-Link has the facility to switch between two internet feeds, the cellular modem and a wired connection into the WAN port on the router. I want to connect a second TP-Link to the WAN port on the first, my issue concerns DHCP. If I leave DHCP enabled on both the whole thing gets into a terrible tangle (as you might expect). It actually sets the first one to a 192.168.1 subnet instead of 192.168.0 and the only way out appears to be a factory reset. However if I disable DHCP on the second router I can't seem to communicate with it, even if I fix my PC's ip to the same subnet. Can anyone help me please
 
If I understand you correctly, then you are aiming to put a second router upstream of the TP-Link 3420 3G..?

If so, it would be necessary for the second router to use different subnet (IP Addressing) from the LAN/WLAN side of the the 3420, so the IP addressing you report actually looks correct.

I suggest you set up router 2 completely stand alone to start with to ensure you have it's communication with the 2nd ISP working reliably, disable it's wi-fi (it will get a real mess if you leave it on) and ensure it's LAN IP address is in a different subnet to the TPLink - so 192.168.1.0/255.255.255.0 should be fine. If you leave router 2's DHCP Server (note server, not client) active then it should be able to supply an IP address to the 3420 WAN interface when you eventually connect them up. However, you could disable DHCP but you'd have to manually configure all the IP settings on the 3420 WAN interface. I'd also prefer not to run the NAT/firewall on router 2, but in SOHO gear you may not have the option to do that. This might lead to issues with "dual NAT" whichhas been known to cause problems.

Once router 2 is talking correctly to it's ISP, disconnect everything else from it and take a cable from one of it's LAN ports to the 3420 WAN port, wait a bit to give DHCP a chance to do it's thing, thence check the 3420 and see what IP address it's WAN port has (it should be a 192.168.1.X) and also take a look at the "System Routing Table" in the 3420. (Post it if you like, and we can tell you whether it looks "good" or not.) If everything has gone to plan, and the faires are with us, there should be entries in the SRT that point 192.168.0.0 to interface "LAN & WLAN," some other gubbins pointing to the 3G ISP and 192.168.1.0 pointing to the "WAN."

However, since this is a "failover" regime, it might be that you don't get a route to the 192.168.1.0 network until the 3G one has failed. It depends on how the 3420 has been designed. So you might need to deliberately fail the 3G interface to force the 192.168.1.0 route to go active. If so, then it means you won't be able to "talk" to router 2 while the 3G link is up. Unfortunately, it's a bit of an "it depends" factor depending on how 3420 has been designed.

For a host connected the to 192.168.0.0 LAN/WLAN, it should have a 192.168.0.X address. It's the routing in the 3420 that should sort out how you get to the 2nd router. If you change a host on the 192.168.0.0 LAN/WLAN to use a 192.168.1.X address, said host basically won't work, so again that's expected.
 
Thanks, that's extremely helpful. As regards fallback, yes the 3420 does have automatic fallback but I don't trust it because what it considers a good or bad connection may not be what I do. However, the switchover can also be forced from the software which is what I propose.
As it happens I have a Netgear GS105E 5 port VLAN switch on the network in question (it is actually two interleaved networks and the VLAN switch lets me cross feed the two networks from each other's cellular internet feed). Although I would rather solve the problem using the TPLink's facility, after I posted that message I had a go with the VLAN switch and it does the job nicely. I am in Africa at the moment and am having to leave in a couple of days so I would rather not tempt fate but leave a working solution for the mo. The reason I would rather use the TPLink is two-fold. Firstly, I have a second network in another town and that one doesn't have a VLAN switch, but also the Netgear firmware is absolutely atrocious and flakey, and due to its remoteness from me I need to have the switch over process as idiot proof as possible.
So when I get home I will experiment with a pair of 3420s in the comfort of my own home. But thanks again, that has really helped clarify my thinking
 
Just thought I'd report back on this. I followed your advice, and it works a treat. I am able to switch backwards and forwards between the two cellular networks flawlessly with no funny issues. Basically, I left router 1 untouched, connected router 2 via a cable to the WAN port of router 1, switched off wifi on router 2, and set router 2 ip address to 192.168.1.?

Thanks again
 
I would probably put router 2 in the DMZ of router 1 otherwise you might end up with strange double NAT issues when you come to do anything fancier .
 

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