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I am trying to understand how BGP works, and is looking at ways to identify if my ISP is having bottlenecks/slowdowns/dos attacks at thier upstream ISPS. I have a bunch of ASN numbers that I can check, but I was wonder if these ASN numbers are tied to physical locations, or can they be at two very different locations with different prefixes for each location. Thanks!

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An ASN (Autonomous System Number) can be announced from multiple physical locations. An ASN can consist of multiple IP blocks. When traffic hits an edge somewhere on the Internet, the "best" path (often the shortest amount of ASN hops) will be chosen to reach that ASN which could be different locations depending on the source.

When you announce certain IP networks from an ASN you are saying "This IP is part of my network". False advertisements, saying you have an IP block that you don't, is a big no-no.

When it comes to an IP block coming from multiple AS numbers this is called a "Multiple Origin Autonomous System" (MOAS) and you can read more about them in An Analysis of BGP Multiple Origin AS (MOAS) conflicts.

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Thanks! That was the answer I was looking for. Than there would be no way to visualize how much traffic each ASN is handling, since they can be separate? –  Craig Apr 25 '12 at 16:16
    
On a more practical note, if you don't trust your ISP to be honest about their upstream saturation find a different one. I don't know of a way to independently get this information (but there might be a way) –  Kyle Brandt Apr 25 '12 at 16:23

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