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