It would appear that ( as of yesterday ) some but not all BGP routes between Cogent and TATA are gone.
From my own observations it seems like all TATA Routes in APAC and India are now not visible from cogent connections/customers.
There's also a report of a cogent support ticket response from reddit (so a small dump truck of salt might be warranted) that suggests it’s a cogent instigated depeering ( https://www.reddit.com/r/networking/comments/1cu13bv/cogent_depeering_tata/ )
Dear customer, For many years, Cogent has been trying to work with TATA on ensuring sufficient connectivity in each global region the networks operate per normal peering practices. Despite Cogent’s repeated requests, TATA has consistently refused to establish connectivity in Asia, taking advantage of Cogent’s good faith efforts while also ensuring sub-standard service to both companies customers. No amount of good will and good faith augments on Cogent’s part has brought TATA any closer to the negotiating table for a resolution to the lack of connectivity in Asia. This one-sided situation has become untenable and as a result, Cogent has elected to start the process of restricting connectivity to TATA.
From bgp.tools point of view, It seems that TATA has routes to cogent in some places, but cogent does not have all TATA India/APAC routes. Also poking around on RIPE Atlas suggests that for a non-zero amount of networks in India the C DNS Root Server that cogent runs is inaccessible: https://atlas.ripe.net/measurements/71894623#probes
Assuming these assumptions are true, there should be around 930 Cogent-Only ASNs and 645 TATA-Only ASNs who now have questionable reachability to each other. My (bgp.tools) visibility for TATA has always been quite slim, especially in APAC, what do other people see on their TATA sessions?