On Jan 14, 2010, at 7:19 AM, Wojciech Dec (wdec) wrote:
2. Figure 3 describes a scenario with no directly attached hosts.
I figured they were clutter in a picture that was trying to describe a
network. If you like, I can add some number of hosts on every LAN and
some multi-homed.
Advertising in RAs any PIO (ULA or other) from the two CPE RTRs would
make sense if the intent was for them to auto-address the routers
themselves, which I think would be of some value in this figure in
terms
of users being able to reach these devices. However I don't quite see
why the number of ULAs actually matters much.
Two points here. The purpose of a ULA is to provide routing
connectivity throughout a domain, just as the purpose of a global
prefix is. We could have every router invent its own ULA and inject it
in routing, but that seems like a waste given that a ULA prefix is by
definition a /48, giving the network the ability to address 65K
subnets within the domain. I'll ask the opposite. What is the value of
having multiple ULAs in a domain apart from specific desires for that
for some administrative reason?
As I said, the issue is distribution and agreement on prefixes, not
the special case of ULA prefixes.
3. If we want to cast the problem in terms of trying to coordinate the
whole customer site to use the same ULA (eg same global-id across the
sire), then this requires some other means beyond RA suppression for
getting the downstream customer routers to also advertise the correct
ULA and suppress their own. Short of inventing RA delegation, going
with
what's already defined (eg DHCP PD) it would make sense to have the
non-suppressed router set itself up as a DHCP DR with the ULA.
That may well be the right way to handle sub-delegation of prefixes
generally. There are issues in that, though - how do we ensure that
the routers that share a LAN share a subnet, as opposed to selecting
different ones? For example, if we presume that the delegating
application tells one of the other routers to use prefix:1:: on
"interface 1" and prefix:2:: on "interface 2", how does it know what
the router it is configuring considers to be "interface 1" and
"interface 2"? The simplest way I can thin of is to have the
downstream router observe the use of a given subnet on a LAN and use
the DHCP message as guidance on what to expect.
So, to put my original question differently: What is the intent of
suppressing a router from sending out an RA with a ULA in the presence
of another router advertising a ULA?
In my opinion, the CPE Router Design Team would like to have at most
one ULA in use in a SOHO/residential domain, and the question is about
achieving that. I think the question is too limited; to my mind, the
question also applies to rational use of global prefixes delegated
from some set of ISPs.
This would appear to be very specific to ULAs as it's clear that any
ISP
delegated prefixes are more or less always meant to make their way to
address the end-hosts, but for ULAs (as in figure 3) that's not
necessarily the case.
-Woj.
prefixes, two of which are global, plus link-local
addressing. On each LAN, apart from local policy that might
have one of the offices using one ISP and the other using the
other ISP or some such thing, you want an IP subnet from each
of those prefixes, and you want all of the routers on the LAN
in each subnet, with the possible exception of the two
ISP-facing routers being in each other's delegated prefixes.
This can be accomplished by manual configuration, by a DHCP
option that to my knowledge is not currently defined, or by
listening to each other's RAs as I suggested. How would you
suggest achieving it?
As to what to do when one loses connectivity to the critical
router, and again considering such a network, there are some
interesting issues. I tend to think the best bet is to use
some form of lease concept rather than making the prefix
suddenly go away throughout, but can be argued into other positions.
On Jan 13, 2010, at 2:27 AM, Wojciech Dec (wdec) wrote:
Perhaps a basic question or two: What is the purpose of the
ULA being
advertised on the shared segment, and is the intent for the "2nd
router"
to auto-config itself an address in the ULA space and begin
advertising that ULA too?
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