ASPATH Loop

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1

Hi， NANOG：

       When I was analyzing the BGP RIB data from RouteViews, I found
that there were aspath loop in many bgp rib entries. For example, in
the file rib.wide.20090301.0319, the peer 202.249.2.169 in AS2497
observed the following aspaths:

3130 29283 3130 2914 2497
3130 7337 3130 2914 2497
3130 34486 3130 2914 2497
3130 38001 3130 2914 2497
3130 13977 3130 2914 2497
3130 25147 3130 2914 2497
3130 5433 3130 2914 2497
3130 26164 3130 2914 2497
3130 34026 3130 2914 2497
3130 31535 3130 2914 2497
3130 14363 3130 2914 2497
3130 29648 3130 2914 2497
3130 27582 3130 2914 2497
3130 39737 3130 2914 2497
3130 33796 3130 2914 2497
3130 15108 3130 2914 2497
3130 9600 3130 2914 2497
......

As we know, BGP instance running on routers don't allow loop in
ASPATH, why they can be seen in RIBs? It's some particular technical
configuration in practice? OR What's wrong with AS3130??

Thanks

Yangyang

2

Look at the WHOIS entry for AS3130, and notice in the comments field:
http://psg.com/as3130/

"Regarding strange announcements by AS 3130 of prefixes in 98.128.0.0/16" is in the big headings on the top of that page.

He is no doubt announcing it with an origin AS of 3130 so no person or router complains about inconsistent origins.

3

OK，I see, Thanks！

4

Multiple discontiguous copies of an AS number in the AS-path can also be
caused by the "original" Cisco IOS "local-as" feature, but in that case the
AS number in the middle would be fixed.
http://bit.ly/7TnCi

And there's always the AS-path prepending. Cisco IOS allows you to insert
anything in the AS path.

Ivan
http://www.ioshints.info/about
http://blog.ioshints.info/