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Here's a good example:

I have a server located in my country, and I purchased from two upstream/transit providers to route my traffic to the world. Upstream A is located in far away region, and Upstream B is located in my country.

So I setup routing preference both for my server, although it worked well for most of my customers, yet there's one local ISP in same country where server is located, and even though it purchases upstream/transit services from same providers A and B, yet the routing for that ISP goes to the out of region to Upstream A rather than the local Upstream B.

Why the ISP routers prefer outside route even though the local upstream provider has/share that same BGP announcement to all its peers including this ISP? What can I do to fix this issue for that particular ISP?

liyan
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    Provider peering is often not free and bases on individual contracts between the connected providers. The shortest path may be more expensive or the provider has no contract to use it. hence the providers do "least-cost-routing" and not "shortest-path-routing". – Robert Mar 17 '20 at 18:45
  • very good answer. In this case, it's out of my control whether that ISP ignore the local routing because of routing expenses to local provider compared to the other one. Is there possibly anyway to make the shortest path even more preferred other than cancelling my contract with the other provider? – liyan Mar 17 '20 at 18:54
  • Are you asking about inbound or outbound routing? Also, how exactly do you get access to those providers (especially those _outside the country_)?... Do you make your own BGP announcements, or does your server-hosting company do that for you? – u1686_grawity Mar 17 '20 at 20:29

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