We're still not aware of the root cause, but it certainly isn't yourselves given some recent testing we've conducted.

Thanks. I'd say you can close this one down, thanks for you and your teams support.

Apologies for the delay in responding. We are taking this up with our ISP as we do not see any traffic coming back from them when capturing on our interface directly connected to their CPE. It would however be beneficial if you can provide me hard evidence that you're responding to our TCP SYNs or ICMP echo requests so that I can give this as proof to our ISP just in case they try to blame you again.

Can you possibly explain why our ISP's interface connected to our network (using IP 84.19.61.194) can successfully ping 91.198.174.192 and 91.198.174.208 (used earlier in testing as suggested by yourselves), whilst all other IP addresses in that same prefix cannot?

This is still very much an issue unfortunately. I have connected my laptop into a layer 2 switch connected to our ISPs CPE and attempted to access en.wikipedia.org. Still seeing no responses after a host in your network, this time it is different to what I've seen in the past (91.198.174.254)

In T279503#6979248, @ayounsi wrote:

Investigation started a while ago on a noc@ thread.

As additional data point, our Netflow (captured at our edge) show that we're getting your initial TCP SYN and replying with a SYN-ACK.
Current theory based one the above as well as previous troubleshooting is that the issue is outside of Wikimedia's network.

Please see the below information from the link you provided.