Anycast usually implies traffic will be directed to the nearest node advertising that prefix. The GLB directors leverage ECMP which provides the ability to balance flows across many available paths.
What Anycast means is just that multiple hosts share the same IP address - as opposed to unicast. When all the nodes sharing the same IP are on the same subnet "nearest" is kind of irrelevant. So the implication is different.
We are using BGP to advertise prefixes for GLB inside the data center to route flows to the directors. In our case all of the nodes are not on the same subnet (or at least not guaranteed to be) which is one of the reasons why we chose to avoid solutions requiring multicast. I expect Joe and Theo will get into more details about that in a future post though.
The article says the L4 layer uses ECMP with consistent/rendezvous hashing. is this vendor implemented or implemented by you using openflow or something similar? How does graceful removal at the director layer work? I know you would have to start directing incoming SYNs to another group, but how do you differentiate non-SYN packets that started on the draining group vs. ones that started on the new group?
If you are using L4 fields in the hash, how do you handle ICMP? This approach could break PMTU discovery because a icmp fragmentation needed packet sent in response to a message sent to one of your DSR boxes might hash to a different box, unless considerations have been made.