Failover using a Single Cisco 1800

Unanswered Question
May 5th, 2007


I have a friend that has dual internet connections coming in for his business. One is a T1 the other is a DSL. Is it possible using the Cisco 1800 with SDM running to setup a failover in case one of his internet connections goes down that outside users can still access webmail, webpage and other things like that?

I have this problem too.
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farkascsgy Sat, 05/05/2007 - 10:21


If you have two ISP lines terminated on one router - DSL, T1 - in case of failure of T1 line I am sure you can't route traffic over the DSL line unless your provider of DSL line route your T1 internet line's IP space. If you use T1 line with provider aggregated address the other provider usually doesn't advertise.

Anyway technically there is no problem to achieve this scenario, you can use static route floating for default route.



bjornarsb Sat, 05/05/2007 - 10:33


Floating static is OK for outgoing traffic, but you need the ISP to setup routing for return traffic. If you have two different ISP its more complex. You need to run BGP and have PI adr. (provider independent IP adr).

I don't think your DSL provider will aggree on mulihomed BGP.



bjornarsb Sat, 05/05/2007 - 10:26


I will not recomend using a wizard for this setup. To make this work properly you need to contact the ISP, at least agree on witch routing protocol you must use.

Assuming you have one ISP for both connections a 1800 router can be setup with failover. The ISP must agree or set your metrics so that the backup path is selected only when the T1 one is down.




jamesreed1 Sun, 05/06/2007 - 08:35

Sorry maybe I did not state the question properly. Two external connections coming into a single 1800 router. The router then connects to the internal network. If the T1 goes down how can the router be setup to still allow internal access to say a website running within the internal network. See the attach graphic, sorry for the lack of drawing skills.

bjornarsb Sun, 05/06/2007 - 22:09


Yes and the answer is still the same.

One question: is it the same ISP providing both the T1 and the DSL ?



bjornarsb Mon, 05/07/2007 - 04:06


Then as I have posted earlier you need to

run Multihomed BGP. And you must get PI addresses from RIPE (europe).



PS. how about changing so that you only have one provider.


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