In some cases, such an incident could lead to massive network degradation across dozens of ISPs. However, it did not. Why?
Because some companies install and maintain their filters properly. And even taking into regard the fact that AS15943 is directly connected to Tier-1 ISPs, they didn’t even notice the incorrect routes. They simply never reached Tier-1s, shrinking in size after each hop.
AS15943 has a very interesting AS-SET for the ISP that is so close to the Internet backbone. It consists of a small number of non-transit stabs. And it was only one ISP from AS15943 Customer Cone, whose prefixes were in AS205310 leak (with AS_PATH 15943 205310 8220 200183). It is interesting because these leaked prefixes were the only one accepted by Cogent (AS174).
as-set: AS-WTNET descr: wilhelm.tel GmbH Norderstedt remarks: wilhelm.tel GmbH Norderstedt members: AS15943 remarks: Spiegel Verlag members: AS199829 remarks: Marquard und Bahls members: AS60264 remarks: Delaval members: AS39268 remarks: Hauni members: AS35391 remarks: Dokumenta members: AS24657 remarks: Hansecom members: AS199273 remarks: HHLA members: AS50943 remarks: FidelioCruise members: AS48355 remarks: Akquinet members: AS35258 remarks: Content Fleet GmbH members: AS200183 remarks: Bit-Serv members: AS200978 remarks: MDEX members: AS200567 remarks: AZV members: AS49473 remarks: GLEICH Aluminium GmbH members: AS204977 remarks: Beiersdorf Shared Services GmbH members: AS205310 remarks: Serverleih members: AS251 remarks: Kolle Rebbe GmbH members: AS3573 remarks: Hermes members: AS29042 remarks: X-ION members: AS60664 remarks: HPA members: AS210303 tech-c: WN136-RIPE admin-c: WN136-RIPE mnt-by: NORDERSTEDT-MNT created: 2012-05-08T08:45:52Z last-modified: 2020-02-13T14:36:19Z source: RIPE
We believe that because only a very few prefixes from AS15943 Customer Cone were leaked, some other Tier-1s (GTT for sure) could also not see this leak by applying prefix filtering based on AS-SETs or other grounds. The remaining part of Tier-1s didn’t see the leak, because they also have a direct connection with AS8220, and all these leaked prefixes were available through this operator in the routes with much shorter length of the AS_PATH than the leaked ones.
All these factors brought the next result. From 90 000 to 9 000 after the first two hops, which explains such a low propagation of such a voluminous route leak; and only 6000 could pass the country border and, for example looking from Moscow, Russia, only 2000 of leaker prefixes made it through. That is a vast difference from a route leak that happened previously, on April 22.
So the AS_SET-based filters work, as we can see in this particular example. We know that for sure with Cogent, GTT and, with high probability, for the rest of Tier-1s.
What is also weird with this route leak is that it consist only from ROA valid routes. So, exclusively the "drop Invalid" policy is not enough for BGP to be safe.