Several IPv6 routers and default gateway choice

[Hiroki Sato]

Victor Sudakov <vas at mpeks.tomsk.su> wrote
  in <20190906035608.GA45741 at admin.sibptus.ru>:

va> Hiroki Sato wrote:
va> > va> Can any IPv6 unicast or link-local address be configured as an anycast
va> > va> address of a router?
va> >
va> >  Yes.  There is no restriction about address scope.
va> >
va> >  You might want to read RFC 4291, which defines Subnet-Router anycast
va> >  address, and RFC 2526, which defines the other reserved IPv6 subnet
va> >  anycast addresses.  In general, the former one can be used for
va> >  routing purpose.
va>
va> Interestingly, RFC4291 says that "All routers are required to support
va> the Subnet-Router anycast addresses for the subnets to which they have
va> interfaces." In practice, I don't observe this. A FreeBSD router does
va> not configure such an address automatically, for example.

 They will be automatically configured when an ipv6_prefix_IF
 rc.conf(5) variable is used to configure a prefix:

----(from rc.conf(5) man page)----
 ipv6_prefix_ed0="2001:db8:1:0 2001:db8:2:0"

 is equivalent to the following:

 ifconfig_ed0_alias0="inet6 2001:db8:1:: eui64 prefixlen 64"
 ifconfig_ed0_alias1="inet6 2001:db8:1:: prefixlen 64 anycast"
 ifconfig_ed0_alias2="inet6 2001:db8:2:: eui64 prefixlen 64"
 ifconfig_ed0_alias3="inet6 2001:db8:2:: prefixlen 64 anycast"

 These Subnet-Router anycast addresses will be added only when
 ipv6_gateway_enable is YES.
----------------------------------

va> RFC2526 is dim, I think it's because of this RFC the last usable address
va> on a subnet ends in ff7f instead of ffff. I wonder if anyone anywhere
va> uses those subnet anycast addresses.

 That is because there are few practical applications of the IPv6
 anycast address other than failover across multiple routers.

-- Hiroki
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