Configuring multiple WAN interfaces on 1841

misahyoun · ‎11-07-2007

I must admit I am a Cisco neophyte and i am trying to configure a 1841 router to support 3 WAN connections. One is a 20mb/s fiber connection, the other two are T1's.

I would like to configure the 20mb/s connection as my primary while the combined T1's would serve as a backup.

I can easily get each one of the connections to function properly, but I don't know how to start on configuring the T1's as backup.

Any hint on how to get started would be greatly appreciated.

Michel

paolo bevilacqua · ‎11-07-2007

Hi Michel,

it's not the easiest job for a 'neophite', but it can be done. Initially, you would set "floating default router" for the T1 connections. That is, you configure like

"ip route 0.0.0.0 0.0.0.0 int serial X/Y 200".

200 is what makes the route to be "floating", that is it will get in effect only when a the default route without such a number, goes away.

The problem with this approach is that you can have cases in which the fiber interface is up, but no actual internet connectivity. To solve this, you need a more advance config called "object tracking" or sometime "IP SLA". It also require policy based routing for reasons that would become clear once getting into the details of such advanced config.

However I suggest you try out first the above before going for the more advanced.

Hope this helps, please rate post if it does!

misahyoun · ‎11-07-2007

Thanks, this is helpful.

How about the NAT configuration in this scenario? Do I need 3 NAT entries and what would they look like?

paolo bevilacqua · ‎11-07-2007

Yes, you do need three NAT entries, and these would all look the same assuming the simpler case in which you are doing NAT (PAT actually) using the public address assigned to the interface:

ip nat inside source list interface X/Y overload

Once you have the backup configuration working, you need to add the 'oer' keyword at the end of the above, so that trasnlations will follow status of routing.

Thanks for the rating and good luck!