Layer 2 based anycast - Kind like GLBP - Research

Douglas Fischer fischerdouglas at
Fri Jul 2 11:02:19 UTC 2021

Hello William!

An ARP Controller to compose a L2 Cluster solution seems a good Idea to a
(I would include ND)

I will try to think a bit on that...

Any suggestions are welcome.

Em qui., 1 de jul. de 2021 às 16:06, William Herrin <bill at>

> On Thu, Jul 1, 2021 at 11:05 AM Douglas Fischer
> <fischerdouglas at> wrote:
> > I'm looking for solutions do deploy some type of selective high
> availability and load balance based on the glue between Layer 2 and Layer 3
> (ARP or ND).
> Hi Douglas,
> Anycast is where you send to one network address and the "nearest"
> single server with that address receives the packet.
> By definition, every piece of equipment in an L2 broadcast domain is
> exactly one hop from every other -- no equipment is "nearer." So
> conceptually, there is no anycast.
> However, L2 domains aren't built with hubs any more; they're built
> with switches. There actually are variable distances between
> equipment, they're just not expressed in the protocols. So, in theory
> you could build an SDN controller for your switches which sets up
> different FIB entries in each switch to select which port receives the
> traffic for the designated "anycast" mac address. But you may face
> limitations where the hardware can't reasonably be programmed to give
> each port its own FIB allowing fine-grained control of which client
> reaches which server.
> Realistically... that approach would tend to be both expensive to
> build and very brittle. There's almost certainly a better way to
> accomplish your goal than trying to invent L2 anycast.
> If you're load balancing IP traffic, another approach might be a
> custom ARP controller which responds to ARP requests with different
> MAC addresses depending on the request source. There's no guaranteed
> timeout for ARP bindings but if you shared around a pool of MAC
> addresses guaranteeing that every MAC address in the pool gets
> assigned to a currently-working server it could work. You just have to
> keep in mind that gratuitous arp absolutely would not work in this
> sort of scenario so you have to have a plan for switching loads
> between servers without it.
> I don't think anybody has built that sort of arp controller (at least
> I haven't heard of one) so you'd have to invent it yourself.
> From what I understand of EVPN, it's about creating something
> equivalent to VLANs across a distributed virtual server
> infrastructure. Basically like what Amazon does under the hood for its
> virtual private cloud. Since you're trying to get the machines to
> appear on the same subnet, not separate them to different subnets, I
> don't think it's what you're looking for.
> Regards,
> Bill Herrin
> --
> William Herrin
> bill at

Douglas Fernando Fischer
Engº de Controle e Automação
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