Multihomed BGP and NAT

Question

I'm aiming to build the following network (might need to open image in new tab due to width):

• Assume those BGP links are different ISPs
  • Same ASN is advertised to both ISPs
  • Sites are interconnected via private line
  • BGP default routes only
  • We have a /24 (public address space)
  • EDIT: I should add that we're already advertising this /24 with BGP at siteA. So I'm essentially looking to add SiteB
• Server 1 is accessible on the internet via NAT

My Question: I'm hoping to expose Server 1 via the same public IP regardless of which ISP a client comes in through. So therefore, I'm assuming the accepted method is to have the same NAT rules on both routers? Is this possible? What are the downsides to this?

Answer 1

This won't work the way you describe it. Each connection has its own public IP belonging to your ISPs. You'll need public IP space (typically called PI space - provider indepentent) which you can advertise via BGP to both your ISP's at the same time.

The setup you describes doesn't have a shared public address between the two connections and your ISPs won't route eachothers addresses, so what you're describing is a server which is reachable through two networks on two different IP addresses. NAT rules can't solve this for you.

Answer 2

The provider owns the /24. We're already using the space at our existing location "site 1"

There isn't a good way around your problem without rights to announce the IP block you have to two different upstreams. You either need:

• Provider independent IPv4 address space (almost impossible to get these days)
• IPv4 Addresses SWIP'd to you
• Work exclusively with IPv6 (probably not an option)

I would ask the owner of the /24 addresses to SWIP them to you. Many providers are happy to do this because they have the potential to off-load some of their backbone traffic to another provider, while still charging you the same rate for your access circuit.

After the provider SWIPs addresses to you and you have your own ASN, you're perfectly in your rights to announce this same block of addresses to another upstream. As I mentioned in the hyperlinked comment, this arrangement tends to work better when you get addresses from a large provider, and you're dual-homing through a smaller provider. The large provider is happy to off-load traffic, and the small provider is happy to have the extra business.

If you're announcing that same /24 block of addresses to different ISPs, I don't see the point in NAT.