I'm considering adding a second 3945 (internet connectivity) router and would like to implement router redundancy. WHat is the best method for an active/active configuration? I would also add a second ISP link to the other router with BGP.
A second router and a second link to the ISP certainly gives you protection against problems that might occur to the first router and to the first ISP connection. It also makes it much easier to schedule maintenance without impacting Internet access. If you get the ISP to accept the second connection in a location different from the first then you also get protection against problems at the ISP POP..
Many people will want to get a second ISP if they get a second connection. But a second ISP creates complexity (whose address space are you using and how do you advertise your address space, do you need separate address translation, what do you do if you have Internet accessible resources on the inside of your network) that is not a factor when the second connection is to the same ISP.
Some very good points (+5). Too often a second link is purchased to a second ISP without considering all the added complexity spedcifically as you say regarding how the address space(s) will be advertised to each ISP and how that addressing will be used on the firewall/router in terms of NAT translation.
Tommy, i am not suggesting you have not thought of these but it is the sort of info we would need to suggest a viable solution. Bear in mind that some ISP will not necessarily want or agree to advertising a small address space from another ISP as this breaks down the aggregating of public addressing throughout the internet.
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