setting up a tunnel behind a home broadband modem.

[Areyouserious]

I want to set up a vpn tunnel between my house and my mates house.

I have purchased two Cisco 2811 routers and have one at each end. 

However each cisco router is behind a home broadband modem.

I'm confused as to how to configure the cisco modems behind already existing home broadband modems, such as whether the home broadband modems need port forwarding on them to the cisco routers or If NAT or PAT is needed ect.

I've been working on this for days trying to set up a vpn with some rasberry pi's but had no luck adn decided to use cisco routers.

Just wondering if anyone might be able to help me with the config on each of the cisco routers?

I had a look at this setup but it doesn't appear to suit the setup i have behind two already setup broadband modems.

https://www.cisco.com/c/en/us/support/docs/routers/1700-series-modular-access-routers/71462-rtr-l2l-ipsec-split.html

Network Map:

LAN-----cisco router-----broadband moem--------internet --------broadband modem-------cisco router-----LAN

[Francesco Molino]

Hi

I’ll explain for 1 side and it will be the same for remote side.

Let’s assume your Internet is 1.1.1.1, you LAN on broadband modem is 192.168.0.0/24 and your LAN on Cisco router is 172.16.0.0/24.

Your wan Cisco router will be in the same subnet as your LAN broadband router, let’s assume your LAN broadband is 192.168.0.1 and your Cisco wan interface is 192.168.0.2.

If you’re able to add static route on your broadband modem, then no need to do nat on your Cisco router and just add a route for 172.16.0.0/24 with 192.168.0.2 as next hop. Your broadband router will nat this subnet when accessing internet. On some ISP router, this isn’t possible to add static route.

Then the solution would be:

 - creating a nat for 172.16.0.0/24 to be natted on your Cisco wan interface 192.168.0.2

You don’t need any port forwarding unless you have inbound services you want to publish on internet like VPN or web services hosted on your Cisco router. In that case, you will need to have a port forwarding rule on your broadband modem and then the adequate nat on your Cisco router.

Is that clear?

[Areyouserious]

Thanks for your reply.

What if I wanted to also have devices using the broadband modem aswell? such as the wi-fi ? rather than having everything routed through the cisco router?

Im only using a couple of devices off the cisco router that's all.

So as to have the tunnel passthrough nat of the broadband router.

[Francesco Molino]

Ok gotcha. If we take examples i given before, if the broadband router is able to do routing then no issue, you will be able to access devices behind Cisco router. If not, you would need to do 1:1 Nat.