According to RFC 3056, 6to4 can only occur over global unicast IPv4
addresses.
ISATAP interfaces can also occur over global unicast IPv4 addresses,
in which
case the ISATAP link would span the entire global IPv4 Internet.
If we allowed both a 6to4 interface and an ISATAP interface to be
anchored
to the *same* global unicast IPv4 address (e.g., "V4ADDR"), it should
be
OK as long as no prefixes derived from "2002:V4ADDR::/48" (i.e., the
/48
prefix itself, or any finer-grained derivative prefix) are configured
as on-link
prefixes on the ISATAP interface. This would keep the "6to4-prefixed
IPv6
Internet" from overlapping with the "everything-else-prefixed IPv6
Internet",
where "everything-else" is a candidate prefix for ISATAP.
I think there may be several possible ways of expressing this, ranging
from
"least restrictive" to: "most restrictive"; below are two possible
examples
that lie at the extremities of the range of alternatives:
Least restrictive:
**************
"An ISATAP and a 6to4 tunnel interface MAY coexist on the same global
unicast IPv4 address ("V4ADDR"), but in this case the ISATAP
interface
MUST NOT configure a prefix derived from " 2002::V4ADDR/48" as
an on-link prefix."
Most restrictive:
**************
"An ISATAP interface configured over a global unicast IPv4 address
MUST NOT configure a prefix derived from "2002::/16" as an
on-link prefix."
(Note that both of these examples still allow for ISATAP interfaces
configured over private IPv4 addresses to configure 6to4 prefixes,
but the ISATAP tunneling would be strictly intra-site in that case.)
Any comments on this?