On March 5th, 2012 at 14:00 UTC (10:00 PM China Standard Time), Renesys observed that Cogent (AS174) and China Telecom (AS4134) appear to have dropped their mutual connectivity. Prior to this time, China Telecom regularly announced about 3000 prefixes to Cogent. After depeering, this ASN adjacency no longer exists in the global routing table.
In addition, the depeering appears to have resulted in an increase in transit from China Telecom to Sprint (in grey below), as expected, since Sprint is China Telecom’s paid transit provider of last resort:
Finally, we can see the impact of the Cogent-China Telecom depeering in our traceroutes. To pick just one example, we see that, prior to the depeering, our traceroute collector in Barcelona was almost exclusively using Cogent to reach China Telecom networks. After the depeering, traceroutes must take paths through one of China Telecom’s transit providers, either Sprint (AS1239) or Level3 (AS3356):
So, what happened? This disconnection will increase China Telecom’s transit costs (mostly to Sprint) and will decrease Cogent’s revenue by losing multi-homed customer traffic destined for China that will now take shorter alternative paths through Level3 or Sprint.
So, is this evidence of another Cogent peering dispute or was there an equipment failure in San Jose, California where the two providers used to connect?
It is important to note that, unlike prior depeerings, there is no complete loss of transit between anyone on earth as a result of this event. Even those brave souls who are single-homed to Cogent will still have paths, because Cogent will peer away their traffic toward Level3 or Sprint, both of whom count China Telecom among their customers.