We recently implemented bogon ASN filtering on all transit/peering edges (see http://mailman.nanog.org/pipermail/nanog/2016-June/086078.html).
There were a few participants we peer with via route servers that had bogon ASN’s in path for various prefixes which we are now rejecting.
I’d suggest that we look at adding similar filtering to the route-servers as well, similar to RFC1918 filters already in place.
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