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

From: Mike Fox <[email-masked]>
Date: Mon, 31 Mar 2003 12:28:46 -0500

Acee Lindem wrote

> On-link prefixes are NOT advertised in router LSAs - only link-LSAs
> and Intra-area prefix LSAs.
> They are considered to be OSPF Intra-area routes.
> I've left out a lot of details - it is probably best to just re-read
> RFC 2740 and ask specific questions.

 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 am asking about interaction between OSPFv3 and the IPv6 Neighbor
Discovery protocol, which is described in RFC 2461 .  Part of the IPv6
Neighbor Discovery protocol is the ability of routers to send ICMPv6 Router
Advertisement messages to on-link hosts, advertising on-link prefixes.
These advertised prefixes can be used in stateless autoconfiguration,
and/or they can also be simply advertised as on-link.

With that background, hopefully  these questions are better phrased and
more to the point:

1. If a router is running OSPFv3, does it still advertise on-link prefixes
via Neighbor Discovery ICMPv6 messages,or does it only use link-LSA's, or
does it use  both?

2. If a host or router running OSPFv3 receives an ICMPv6 Neighbor Discovery
message advertising an on-link prefix, and that prefix is not also received
in a link-LSA, does it treat it as OSPF internal or OSPF external?

Mike

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From: Acee Lindem <[email-masked]>
Date: Mon, 31 Mar 2003 13:02:01 -0500

Hi Mike,

You're right - I was confused by initial E-mail. I'll give you
my opinion on the interaction between ND and OSPFv3. I'd be very
interested in the opinions of those who have followed the IPng
discussion more diligently than myself.

> via Neighbor Discovery ICMPv6 messages,or does it only use link-LSA's, or

An IPv6 router must advertise prefixes in ND router advertisements.
If OSPFv3 is configured on the interface, it also must advertise its
prefixes in a link-local LSA.

> in a link-LSA, does it treat it as OSPF internal or OSPF external?

Hmmm, I'd call this a misconfiguration. I'd log at least an INFO or WARN
message. Since you're a host (and not a router) you listen to ND RAs and
install the corresponding RA routes. In this case, the corresponding route
type will not be OSPFv3 at all. I'd think it would be an ND stateless
auto-configuration route. If you redistribute the route back into
OSPFv3 and re-advertise it, then it should probably be advertised in
an OSPFv3 external LSA.

--
Acee

---

From: Yasuhiro Ohara <[email-masked]>
Date: Tue, 1 Apr 2003 03:01:56 +0900

Hi Mike,

> 2740, and as far as I can tell, it does cover the interaction I am asking

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

> via Neighbor Discovery ICMPv6 messages,or does it only use link-LSA's, or

I guess that all depends on the configuration of the router. A router
can know his on-link prefix either Link-LSA or RA, or both. Even if
the router receives both Link-LSA and RA, there won't be any problem,
at least protocol level.

One interesting case will be when the router receives RA,
and the router's interface is "passive" or something that
makes OSPFv3 to advertise attached address prefix on that interface.
Then the router will advertise the prefix in the Intra-Area-Prefix-LSA
referring the router's Router-LSA.

> in a link-LSA, does it treat it as OSPF internal or OSPF external?

Usually OSPFv3 do not advertise the address prefix on the interface
which is not included in OSPF domain (in other words the interface
that does neither speak OSPF nor form OSPF adjacency), I believe.
So in your case, OSPF will have nothing to do with the address prefix,
except for the case I mentioned above.

regards,
yasu