Several IPv6 routers and default gateway choice

[Hiroki Sato]

Victor Sudakov <vas at mpeks.tomsk.su> wrote
  in <20190830021228.GA66465 at admin.sibptus.ru>:

va> Dear Colleagues,
va>
va> If a FreeBSD host receives router advertisements from several IPv6
va> routers, on a LAN segment, which would be chosen as the default gateway?

 One reachable router will be selected based on the default router
 preference value in RA, which is defined in RFC 4191.  If all of the
 routers have the same preference, the first one will win.

va> Can this be used for failover?

 Yes.

 Another way to realize failover is to use a common anycast address on
 multiple routers. For example, a router is always able to have
 fe80::/64 as an anycast address like this:

  router# ifconfig igb0 inet6 fe80::/64 anycast

 and you can simply configure fe80::/64 as the default router on the
 hosts.  Multiple routers with the same fe80::/64 can coexist on the
 same segment, and hosts will choose one of them with no further
 configuration.  A caveat in this case is that the first router always
 wins and there is no knob to set the preferences across the routers
 with the same anycast address configured.

-- Hiroki
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