TFTP over anycast

Sat Feb 24 04:32:33 UTC 2024

On Fri, Feb 23, 2024 at 6:34 PM Ask Bjørn Hansen <ask at develooper.com> 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.
Devices behind a load balancer are not "anycast," since the load
balancer explicitly decides which machine gets which transaction. Even
with clever load balancers like Linux Virtual Server in "routing" mode
where the back-end servers all share the virtual IP address, that's
load balancing, not anycast routing.

An IP is not "anycast" unless it moves via anycast routing. Anycast
routing means it's announced into the _routing protocol_ from multiple
sources on behalf of multiple distinct machines.

In their readme, they comment that their load balancer replaced
attempts to use anycast routing with equal cost multipath. That makes
good sense. 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.
All the DHCP renewals would very suddenly be going to the wrong DHCP
server. Where anycast works, it works because ECMP only rarely comes
into play.

Regards,
Bill Herrin

--
William Herrin
bill at herrin.us
https://bill.herrin.us/