The system Ask is describing is the traditional method of using anycast to geographically load-balance long-lived flows.  The first time I did that was with FTP servers in Berkeley and Santa Cruz, in 1989. 

I did a bigger system, also load balancing FTP servers for Oracle, their public-facing documentation stores, with servers in San Jose and Washington DC, a couple of years later.   A couple of years further on and the World Wide Web was a thing, and everybody was doing it. 
    
                -Bill


On Feb 24, 2024, at 7:38 AM, Ask Bjørn Hansen <ask@develooper.com> wrote:



On Feb 23, 2024, at 20:32, William Herrin <bill@herrin.us> wrote:

The relay server `dhcplb` could, maybe, help in that scenario
(dhcplb runs on the anycast IP, the “real” DHCP servers on
unicast IPs behind dhcplb).

Although they used the word "anycast", they're just load balancing.

The idea is to run the relays on an anycasted IP (so the load balancer / relay IP is anycasted).

[….] Relying on ECMP for anycasted DHCP would be a disaster
during any sort of failure. Add or remove a single route from an ECMP
set and the hashed path selection changes for most of the connections.

Consistent hashing (which I thought was widely supported now in ECMP implementations) and a bit of automation in how announcements are added can greatly mitigate this.



Ask