On 1/27/15 5:45 AM, Song Li wrote:
Hi everyone,
Recently I studied the BGP AS path looping problem, and found that in most cases, the received BGP routes containing local AS# are suspicious. However, we checked our BGP routing table (AS23910,CERNET2) on juniper router(show route hidden terse aspath-regex .*23910.* ), and have not found such routes in Adj-RIB-In.
Updates with your AS in the path are discarded as part of loop detection, e.g. they do not become candidate routes. https://tools.ietf.org/html/rfc4271 page 77 If the AS_PATH attribute of a BGP route contains an AS loop, the BGP route should be excluded from the Phase 2 decision function. AS loop detection is done by scanning the full AS path (as specified in the AS_PATH attribute), and checking that the autonomous system number of the local system does not appear in the AS path. Operations of a BGP speaker that is configured to accept routes with its own autonomous system number in the AS path are outside the scope of this document. in junos neighbor { ipAddress | ipv6Address | peerGroupName } allowas-in number where number is the number of instances of your AS in the path you're willing to accept will correct that.
We believe that the received BGP routes containing local AS# are related to BGP security problem.
You'll have to elaborate, since their existence is a basic principle in the operation of bgp and they are ubiquitous. Island instances of a distributed ASN communicate with each other by allowing such routes in so that they can be evaluated one the basis of prefix, specificity, AS path length and so forth.
Hence, we want to look for some real cases in the wild. Could anybody give us some examples of such routes?
Thanks!
Best Regards!