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The question as following: Suppose I have 3 POP (Points of Presence) points, they have the same IP address and located at different countries, such as: USA, Japan, England. The three countries serve the whole world internet users. We hope all of USA area requests go to USA POP point, all of Japan area requests go to Japan POP point and all of European area requests go to England POP points.

But the reality is not, we found may Japan requests go to England, further more some USA requests go to Japan.

So now, my solution is to add AS-path prepending for the three points(ISP AS). For example if I add 1 to USA, add 3 to Japan and add 5 to England. Then there would be fewer requests go to England, and more requests go to USA, because England as-path become longer while USA become shorter. But it also might cause the European requests also left England and go to USA.

If the as-path prepending range is 1-5, then there are 5*5*5 combinations, such as {1,1,1},{1,2,3},{2,5,1}...

So I'm wondering if there is a good algorithm for selecting the best combination for the problem.

Actually I have 20 POP points instead of 3 in our real environment.

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    It's very difficult to control routing on the Internet. There may be ISPs in Japan (for instance) that are better connected to the US than the rest of Japan or Asia. So first question: how important is keeping the traffic on the same side of the ocean, really? Second: Is latency really an issue? Third, have you considered alternatives to anycast, such as CDN or DNS-base solutions?
    – Ron Trunk
    Commented Aug 7, 2018 at 2:55
  • hi Ron, thank you, I'm using anycast, actually this question is about anycast.
    – Jack
    Commented Aug 7, 2018 at 21:05
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    Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin ♦
    Commented Dec 25, 2018 at 9:14

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As Ron Trunk already indicated in a comment, using AS prepending to achieve this can be hard, or maybe even impossible. Other networks are free to totally ignore your prepends and just chose their best path based on their local preferences. Only if you have a lot of BGP peers at each of your PoP's you may be able to influence some of the inbound traffic by setting AS prepends for each peer individually, but still, you have very little control.

The common solution is to either use a CDN, or to use a form of global load balancing, where the DNS server answers with the IP address of the cluster it thinks is best suited, or where a HTTP proxy redirects the client after the first visit to a more local cluster.

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