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Summry question:

If ISP A only gives a BGP Default and ISP B gives me the full table, can my router use the information it receives from ISB B to know what IP's are easier accessed through ISP A?


I'm trying to set up BGP sessions with two providers, let's call them A and B. Those two providers are the two biggest ISP's in my Country, each of them holding almost equal parts of the business and residential broadband market in the Country.

Provider A wants to impose some artificial limitations on the BGP session by only giving me a Default, while provider B imposes no such limitations and it's willing to give me the full table. I've got comparable connections to both providers; When talking to broadband users that get connections from A or B, I'd like to use the connection to the respective ISP. In other words, I do not want a main-connection / backup-connection kind of setup, I want to use both connections full-time. Since people interesting in visiting my web sites are almost exclusively residents of my Country, being able to talk to them directly through their respective ISP's should result in fairly good balancing between my two connections.

If both providers would give me full tables, the balancing I want would be automatic because the AS-Path to clients of provider A would always have a length of 1 over the connection to provider A and >1 over the connection to provider B, and vice-versa. If both providers would only give me defaults, I assume there would be no way to differentiate customers of A and B, and I would end up with a main-connection / backup-connection kind of setup. But I'm in the sittuation where one provider would only give me a Default while the other would give me the full table!

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up vote 5 down vote accepted

You could possibly filter the routes from provider B, to remove routes that have more than N AS entries in the AS-path, then preferring the default route from provider A.

However, it would be much better if provider A could give you (at least) a BGP feed of ASes that they connect directly to.

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It's good to know some hacks are possible, even if I don't get both providers to give enough information. – Cosmin Prund Nov 2 '11 at 12:24

If you get a default only from A, and a full feed from B (and then some of those routes have A in them), you could write a policy that up-preferences the routes from B that have A in the as-path out the A pipe. Keep in mind that this can be somewhat dangerous, as you have the potential to blackhole traffic. In actuality, getting a default from anyone can lead to traffic blackholing, as you have no idea what the router on the other side of the connection can get to.

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As I see it any reliance on defaults, even for only part of the traffic, can cause "blackholing". None the less I don't have much of a choice. In the end I got partial feeds from both providers and no full-feed. The provider willing to provide the full feed finally told me I need a higher bandwidth connection OR I need to pay for the full feed. So I selected the partial, nation-wide feed, because I got that for free. – Cosmin Prund Nov 24 '11 at 7:09

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