Subject: Re: OSPFv3 nexthop

From: Sina Mirtorabi <[email-masked]>
Date: Sun, 1 Dec 2002 03:40:06 -0800

Yasu,

>
> sina> having global address in Link-LSA is irrelevant to next hop
> sina> calculation, the fact of carrying global IPv6 address
> in Link-LSA
> sina> is just related to announcing all global Ipv6 address of a
> sina> multi-access link to DR so that it will announce them in its
> sina> intra-area-prefix LSA
> sina>
> sina> Link-local address is the only address to be always
> available ( as
> sina> Acee said ) therefore this address can always be used to reach
> sina> your nexthop and you should not need any other address
> even a node
> sina> can have multiple IPv6 address

Yasu> Having a global address in Link-LSA (specifically in
Yasu> 'Link-local Interface Address' field) is *not* irrelevant. It
Yasu> will be used as the actual nexthop in SPF calculation.

First of all as its name says "Link-local Interface Address" field
contain a link local address and *Not* a global IPv6 address, it is
clearly mentioned in the RFC

---
Link-local Interface Address
      The originating router's link-local interface address on the
      link.

...

Link-LSAs have three purposes: 1) they
   provide the router's link-local address to all other routers attached
   to the link and 2) they inform other routers attached to the link of
   a list of IPv6 prefixes to associate with the link and 3) they allow
   the router to assert a collection of Options bits to associate with
   the Network-LSA that will be originated for the link.

...

A router learns the link-local addresses of all other
   routers attached to its links, and uses these addresses as next hop
   information during packet forwarding.

----

I think it is clear enough from the above that the global address in the
Link-LSA is used for announcing those prefix to the DR and therefore is
irrelevant to nexthop calculation which use only link-local address;-)

Last we do not really need Link-LSA for nexthop calculation ( as it is
mentioned in the RFC ) except for NBMA, since we would learn them
through Hello reception and the source IPv6 address of this packet *is*
a link-local address

Yasu> It is possible for an implementation to recognize a Link-LSA
Yasu> which has a global-address in 'Link-local Interface Address'
Yasu> field as an invalid Link-LSA. It is just because RFC2740
Yasu> describes that way (or the field name indicates that way).

I don't think so, the filed name says clearly what is the value of this
field otherwise it would have stated that it can be an IPv6 address

Yasu> I agree that if OSPFv3 implementation allows, having global
Yasu> address as nexthop will not be a problem.

Yasu> Route table lookup may ensue further to find link-local
Yasu> address for it, or NDP(or some other L2-L3 address mapping
Yasu> mechanism) will work directly on the global address to derive
Yasu> its corresponding L2 address.

The question is Why should I ever need to set the nexthop to a global
Ipv6 address ? If you tell me that then I would agree that RFC need to
mention that the nexthop can be set to a global IPv6 address.

After all you want to send your packet to your _nexthop_ so your
neighbor link-local address is the Ipv6 address that is always available
and least likely to change

Sina

> sina> If you are referring to FA setting, although if it is
> necessary to
> sina> have a global Ipv6 address for FA setting this address is only
> sina> used to forward a packet to, but this does Not mean that the
> sina> nexthop is the FA. A router while trying to install a
> route to the
> sina> external with FA set needs to just know its nexthop
> which would be
> sina> a link-local address ( even if the ASBR setting the FA
> is directly
> sina> connected to this router )
>
> Agree, FA will be used to derive actual nexthop, which will
> be a link-local address in most cases.
>
> regards,
> yasu
>

---

From: Yasuhiro Ohara <[email-masked]>
Date: Mon, 2 Dec 2002 18:21:47 +0900

sina> First of all as its name says "Link-local Interface Address" field
sina> contain a link local address and *Not* a global IPv6 address, it is
sina> clearly mentioned in the RFC

Okay, I thought the original question was "Why do we restrict next-hop
to be link-local ?" (interpret by me as "What will happen when we use
global address as nexthop by advertising a global-address within
Link-local Interface Address field in Link-LSA ?")

sina> The question is Why should I ever need to set the nexthop to a global
sina> Ipv6 address ? If you tell me that then I would agree that RFC need to
sina> mention that the nexthop can be set to a global IPv6 address.

Yes, we don't need nexthops to be global, as far as I know, even in
the FA case.

I wanted to mention that technically nexthop address advertised with
Link-LSA (thus used as actual nexthop) could be global address. I
agree that it is simpler to restrict nexthop to be link-local address,
and I agree rest of your message.

regards,
yasu