Re: IPv6
It gets complicated, but then proper multi-homing always has been. Issue is basically if you have PA IP addresses (Provider Allocated), then the provider advertisers the aggregate, then other providers base their route filters off that. So you can't advertise a more specific PA address via another ISP.
So 'solution' is to either get PI (Provider Independent) addresses, but then you need to route those via multiple ISPs.. Which then (generally) means BGP, which means getting an ASN (Autonomous System Number) and creating/maintaining RADB objects.
And from the sounds of things, wouldn't help in this case if Centurylink was advertising L3 routes externally, but they weren't reachable internally.. So traffic would still go towards Centurylink, and then end up in an internal bit bucket inside their cloud.
It's possible to kludge it locally with 2 ISP connections, and run a router/traffic manager to prefer 1 link over another, but without BGP from upstreams, you wouldn't have a routing table to do any fancy route selection, and wouldn't be able to force return path. So you could prefer outbound over 1 link, but return traffic may still end up trying to go via L3/Centurylink and fail.
Biting the hand that feeds IT
