Several IPv6 routers and default gateway choice

[Hiroki Sato]

Victor Sudakov <vas at mpeks.tomsk.su> wrote
  in <20190901141047.GA56954 at admin.sibptus.ru>:

va> Hiroki Sato wrote:
va> >  Another way to realize failover is to use a common anycast address on
va> >  multiple routers. For example, a router is always able to have
va> >  fe80::/64 as an anycast address like this:
va> >
va> >   router# ifconfig igb0 inet6 fe80::/64 anycast
va> >
va> >  and you can simply configure fe80::/64 as the default router on the
va> >  hosts.  Multiple routers with the same fe80::/64 can coexist on the
va> >  same segment, and hosts will choose one of them with no further
va> >  configuration.  A caveat in this case is that the first router always
va> >  wins and there is no knob to set the preferences across the routers
va> >  with the same anycast address configured.
va>
va> Thank you Hiroki, this was very informative and useful.
va>
va> Can any IPv6 unicast or link-local address be configured as an anycast
va> address of a router?

 Yes.  There is no restriction about address scope.

 You might want to read RFC 4291, which defines Subnet-Router anycast
 address, and RFC 2526, which defines the other reserved IPv6 subnet
 anycast addresses.  In general, the former one can be used for
 routing purpose.

va> address of a router? Is this a replacement for VRRP and carp(4)?

 Mostly yes.  VRRP and CARP use a virtual IP address and active
 heartbeat packets to detect unreachability between the member NICs.
 They support fine-grained configurations such as heartbeat interval,
 password, and preference.  On the other hand, anycast IPv6 default
 router uses built-in unreachability detection of the IPv6 core
 protocol.  No control communication happens between NICs with the
 same anycast address.  The client will pick up one router and use it
 as long as it is reachable.  It is only for IPv6, of course.

 So the anycast address just works if you do not need password or
 control of the master selection.  For master selection, router
 advertisements with different preference values can be used in
 combination with anycast addresses.

-- Hiroki
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