BGP/Multi Homing

Well you're right, DNS is definitely not the answer - take that from someone who has run multi-homed ISP networks, and now does DNS for living.

What was the £80k quote for - just BGP and an additional transit feed, or for the necessary Cisco routers too? The 2800s you're currently listing probably aren't capable of running a full routing table - there's currently over 200k routes in the global BGP4 table, and that takes a lot of router memory.

It's a couple of years since I was doing this for real, but actually getting BGP from transit suppliers shouldn't be expensive - indeed the larger scale suppliers expect to offer BGP as part of the service, particularly if you're taking 100+ Mbps.

Also, where's the current main data center? You don't necessarily need massive diversity - my network originally had two DCs in London - one in the city and one in Docklands, about 10km apart. That's far enough to rule out most any natural disaster.

If you have both sites in London there's a number of companies that offer cheap ethernet links between the many data centers in the city. One of the most well regarded is Datahop - they do 1 Gbps links between sites for about £4k per annum.

Similarly for the backup site if you only want the second transit link to be used in emergencies then I've seen stupidly low prices from the likes of Cogent for £5 per Mbps pcm. I wouldn't use them as a primary, but as a transit of last resort they're worth considering.

For using multiple outgoing connections, all you need is something to route the traffic appropriately. There are a number of free linux/BSD router distros that will work fine, such as m0n0wall.

However, to share the same set of public IPs on multiple incoming connections, ISP participation is required.

This is indeed referred to as multi-homing, and requires the use of BGP, an assigned AS number, and ownership of an IP space.

This is not often feasible for such small IP spaces; you may be able to get a redundant link from one ISP, in which case they will take care of the multi-homing, but that doesn't protect you against that ISP going down...

An alternative is to host your critical services in a datacenter, which will be redundant in all respects (power, cooling, hardware, connectivity) - but you will have to compare costs.