why are there several times repeated AS number appearing in certain AS path? [closed]

StackOverflow https://stackoverflow.com/questions/18470552

  •  26-06-2022
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Question

I try to use the bgpdump shared library to read routeview's MRT format file. In the output, I found the following AS path:

1299 9318 38091 38091 38091 38091 38091 18313

The are many of them and this is only one example.

My question is why the ASN 38091 appears several times? Is it some kind of error? If it is not, why does it happen?

Was it helpful?

Solution

You are not seeing an error; the practice of adding your own AS number multiple times is called AS Path Prepending.

Some multihomed eBGP autonomous systems use AS path prepending to ensure that at least one of their peers are less preferred, and they make some (or all) routes announced to that peer less preferable with AS Path Prepending. eBGP selects paths based on the shortest AS path, therefore AS Path prepending makes those announcements less preferred than announcements that were not prepended.

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