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25 January 2011

robust internet connection 1/2

Until recently, my overkill home network had two routers with DSL cards in them. An 877W and a 2621XM with a WIC-1ADSL.

The 870 served as the outside router, providing an Internet connection via aDSL to my ISP. I have OSPF running between the two routers, including default route distribution (default-information originate) from the outside router, informing other routers about an Internet access path.

Typical setup. Cisco 877W is Internet gateway and responsible for NAT. Traffic is routed via 2621XM.

As I was fiddling with routing protocols and had multiple DSL cards, one question came up: could I have such a setup that if I swap my DSL cable from one router to the other, the user network can keep accessing the Internet without having to configure/reconfigure anything?

I thought so, and I can! What's required?
  1. A routing protocol to update the routing tables of both routers if such a change would happen. done: OSPF. But now the core router would also have a default route to distribute should it become the Internet uplink (so default-information originate on it too);
  2. Redundant NAT configurations, so both routers perform address translation in the same way: can do;

Alternative setup. Cisco 2621XM becomes Internet gateway, also responsible for NAT.

So after a little while, I got it going. Sadly, the WIC-1ADSL is far far from being as good as the 877W's DSL chipset, so Internet performance suffers a good bit. However, the exercise here is technological and it is about redundancy and/or robustness (albeit partially manual, as I have to switch the cable between routers).

... to be continued.