Why can't a customer get 2 different public IPs, 1 per ISP?

Hi Rene, please help me understand the basic concept behind why we need BGP.
Here’s an excerpt from your tutorial:
“In my previous examples we got public IP addresses from our ISP. Now I’m connected to two different ISPs so what public IP addresses should I use? From ISP1 or ISP2? If we use public IP addresses from ISP1 (or ISP2) then these servers will be unreachable once the ISP has connectivity issues.”

My question:
Customer’s default gateway has dedicated links that connect to each ISP. Why can’t customer advertise 2 default routes using 2 different Public IPs received from the 2 different ISPs? So, even if one ISP fails, internet traffic will still reach the customer through the other ISP. Customer can do NAT/PAT at the default gateway.

  1. So, why should customer get their own Public IP space?
  2. Even if the customer gets their own Public IP space from IANA, why can’t they do default routing there too? Why do they need BGP at all, other than the fact that they can play around with BGP path attributes?

Hello Jo

If you have two ISPs for the purpose of obtaining redundant Internet connectivity for your internal users, then BGP is not necessary. BGP is only necessary when you have some service running on your enterprise network, such as a Web Server, that you want Internet users to be able to access from the Internet.

Now if you have two ISPs, then each of those ISPs will have a specific external IP address, in two separate ranges, as each ISP is given its own range. Now your web server will have a DNS such as www.yourdnsname.com. Which of the two IP addresses will you register with the DNS service? That of ISP1 or ISP2? If you use that of ISP1, then if that goes down, there is no way to let the rest of the Internet know that in order to reach the web server, go through ISP2, unless you use BGP.

One solution is to get your own registered IP address range and use it in your DMZ or your datacentre and advertise this range via both ISPs. That way, the Internet will be informed of the single IP address that corresponds to your DNS name and even if one ISP goes down, the advertising will continue via the other.

You can use NAT/PAT to translate between the ISP’s public IP address and the private address you’re using for your web server, however, you still have the problem of which external IP address will you use as the definitive address that corresponds to your web server.

I hope this has been helpful!


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Hi Lazaros,

Brilliant explanation! Thank you so much for the clarification. Much appreciated.