I don't see anything in the RFC about the behavior of routers in this
context; in point of fact, I don't see anything about routers coming
up and finding each other. I would interpret that as "routers are
expected to be configured, manually or dynamically, as opposed to
determining prefixes and addresses dynamically".
It seems to me that it is very rational to consider a router as
getting an address on an interface in three possible ways:
- manual configuration
- DHCP [RFC 3315]
- via a Router Advertisement from another router
manual configuration takes precedence over dynamic configuration, and
dynamic configuration over an RA. But if a router has neither of the
first two, then I would agree that on that interface it is operating
as a host until something changes that.
By that logic, it should send an RS, and with the resulting RA it
should configure itself with both a prefix for the interface and an
address within the LAN. Absent the prefix for the interface, it would
have to forward datagrams to the other router in order to know they
belonged on the local LAN. Having "inherited" that configuration, if
you will, there is something it should *not* do: it should not issue
an RA with that prefix while it is in contact with said other router,
as doing so is redundant. It should only send RAs if it loses contact
with the original router advertising the prefix.
Note that this is suddenly not specific to ULAs; it is also true of
global addresses. If I have more than one router on a LAN and one is
responding to RS's with RAs, the others should not do so on that
interface, but should inherit their prefix from the router that is
responding.
On Jan 12, 2010, at 9:50 AM, Ole Troan wrote:
hi,
a question arose from work I'm doing with the BBF and their CPE
requirements document (TR-124/WT-192). an issue has been raised with
regards to a requirement about CPE routers automatically offering
ULA addresses on the LAN. in the case of multiple CPE routers on a
link, the suggestion is the following two requirements:
LAN.ADDRESSv6. 3 The device MUST send a Router Solicitation to the
LAN, to determine if there
are other routers
present. MUST
LAN.ADDRESSv6. 4 If the device determines other routers are present
in the LAN, and that another
router is advertising
a ULA prefix, the device MUST be configurable to
automatically use
this information to decide not to advertise its own
ULA prefix. MUST
any opinion on these requirements and how they compare with expected
behavour as specified in RFC4861?
cheers,
Ole
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