> The issue isn't that they needed a backup to cloudflare. The problem was they only have a single internet provider at their datacenter, so they couldn't communicate with Cloudflare.
That's not the issue. With Cloudflare MagicTransit, packets come in from Cloudflare, and egress normally. They were able to get packets from Cloudflare, but egress wasn't working to all destinations. I wasn't able to communicate with them from my CenturyLink DSL in Seattle, but when I forced a new IP that happened to be in a different /24, because I was seeing some other issues too, the fastmail issues resolved (although timing may be coincidental). Connecting via Verizon and T-Mobile, or a rented server in Seattle also worked. It's kind of a shame they don't provide services with IPv6, because if 5% of IPv4 failed and 5% of IPv6 failed, chances are good that the overall impact to users would be less than 5%, possibly much less, depending on exactly what the underlying issue was (which isn't disclosed); if it was a physical link issue, that's going to affect v4 and v6 traffic that is routed over it, but if it's a BGP announcement issue, those are often separate.
That's not the issue. With Cloudflare MagicTransit, packets come in from Cloudflare, and egress normally. They were able to get packets from Cloudflare, but egress wasn't working to all destinations. I wasn't able to communicate with them from my CenturyLink DSL in Seattle, but when I forced a new IP that happened to be in a different /24, because I was seeing some other issues too, the fastmail issues resolved (although timing may be coincidental). Connecting via Verizon and T-Mobile, or a rented server in Seattle also worked. It's kind of a shame they don't provide services with IPv6, because if 5% of IPv4 failed and 5% of IPv6 failed, chances are good that the overall impact to users would be less than 5%, possibly much less, depending on exactly what the underlying issue was (which isn't disclosed); if it was a physical link issue, that's going to affect v4 and v6 traffic that is routed over it, but if it's a BGP announcement issue, those are often separate.