Bgp – Multihomed BGP and NAT

bgpnat;

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

sample network

  • 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?

Best Answer

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.

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