Subject: Re: OSPFv3 and IPv6 on-link router advertisements

From: Mike Fox <[email-masked]>
Date: Mon, 31 Mar 2003 15:09:47 -0500

Yasuhiro Ohara  wrote:
> Mike Fox wrote

> >  I don't think my question was precise enough.  I have been through RFC
> > 2740, and as far as I can tell, it does cover the interaction I am
asking
> > about.

> I don't think so, which part is it mentioned ?

You're right Yasuhiro, it was a typo in my last message.  It should have
said "...it does NOT cover the interaction..."  If it did cover it, I would
not need ask this question :)

Acee wrote:

> Anyway, my opinion is that routers should rely on the routing
> protocols (e.g., OSPFv3) to learn about on-link prefixes and
> that neighbor discovery RAs are primarily for hosts.

Unfortunately the distinction between host and router is not so clear-cut
for us.  My product is a hybrid of a host and router.  For purposes of this
question,  it receives ICMPv6 Router Advertisements and never sends them.
On the same interfaces,  it also participates fully in the OSPF autonomous
system.

 As we implemented IPv6, we initially shipped our IPv6 implementation
without support for a dynamic routing protocol, relying solely on static
routes and router advertisements for IPv6 routing. However, IPv6 dynamic
routing is a requirement for our customers, so now I am working to
understand what's needed to add OSPFv3.   Since we already have OSPFv2
implemented most of it is pretty straightforward, but we are puzzling over
how the Router Advertisement prefixes and OSPF Autonomous System will
interact.

It seems like OSPFv3 and Neighbor Discovery were not designed to work
together -- neither RFC mentions the other. The assumption that some people
have made in this discussion, and maybe also the assumption made by the
designers of these protocols,  seems to be that you support one or the
other.   But we support both.  We need to receive the Router Advertisements
so we can do stateless autoconfiguration.  But we will also support OSPFv3.

If a router that sends RA packets also sends link-LSAs for the same
prefixes, then that answers the question -- obviously, any prefix received
on a link-LSA will be OSPF internal.

However, if the router does not send a link-LSA for every on-link prefix
that it advertises via RA (or if there is a router on my LAN that is
sending Router Advertisements but NOT running OSPFv3) then I need to decide
if a prefix learned only via RA is OSPF internal or OSPF external.  There
are good arguments for both answers.

For OSPF internal, one could argue that the prefix is assigned to a link
that is internal to the OSPF AS (since I'm running OSPF on that link), so
it should be internal.   In this view, RA is just a lower-level method for
the TCP/IP stack to  learn its interface prefixes. Futhermore, if it is
OSPF internal than I need to include it on my link-LSA.

For OSPF external, one could argue that the prefix was learned via a
routing protocol other than OSPF.  In this view Router Advertisement is
another routing protocol and therefore a different autonomous system, and I
would need to advertise it on an AS-external LSA. .

Mike

---

From: Acee Lindem <[email-masked]>
Date: Mon, 31 Mar 2003 16:31:04 -0500

> would need to advertise it on an AS-external LSA. .

Mike,

There are really at least a couple issues here. First off, you may
have 2 routes but only one is active. So, you need to have the concept
of adminstrative distance (or preference) to determine whether you
install the OSPFv3 route or the ND auto-config route. IMHO, the default
adminstrative distance should prefer the ND route over the OSPFv3 route
since it is somewhat analogous to a connected (or direct) route.

Once you've determined which route to install, the next decision is whether
or not to re-advertise the route. If you install the ND route, you need to
determine 1) Whether or not you will include it in your own link-LSA.
2) If so, then it will be advertised to others as an OSPF route but it
will remain an ND route to you. 3) If not, then it could be optionally
redistributed into OSPFv3 and advertised as an AS external route. If you
install the OSPFv3 route, you would certainly not re-advertise it in your
own link-LSA.

If your box is acting as a router, you should probably include a configuration
knob to disable listening to RAs on the interface.

Make more sense?

Acee

--
Acee