Subject: Re: OSPFv3 nexthop

From: "Manral, Vishwas" <[email-masked]>
Date: Fri, 29 Nov 2002 08:11:21 -0500

Acee,

>> I was going thru the OSPFv3 RFC which in section it states that
>>
>> "In IPv6, the calculation of the next hop's IPv6 address (which
>> will be a link-local address) proceeds along the same lines as
>> the IPv4 next hop calculation (see Section 16.1.1 of [Ref1])."
>> I think this is not correct. After carefully reading the RFC I
>> figured out that Link-LSA's can have global scope IPv6 addresses
>> too. I see no reason why we cannot have global addresses as next
>> hop.
> You are guarenteed to have a link local address on all OSPFv3
> interfaces while there is no guarentee that you have a global
> address. I believe this is the rationale for using the
> link local.
I agree to the fact that all IPv6(not OSPFv3 in context of the IPv4
discussion;-)) interfaces will have link-local addresses, besides I am ok
with the fact of using Link-local addresses for nexthop. However I think
saying that nexthops "will" be link-local addresses would be wrong, we can
have global addresses in Link-LSA's and as nexthop(also the reason below)

>> Besides for some conditions
>> of redistribution we may require global scope addresses for nexthop.
> Are you trying determine whether a third party next hop can be
> advertised by matching global subnets?
When redistributing OSPF routes into BGP, BGP may use the nexthop learned
from the OSPF to advertize in its route. I guess, we cannot send link-local
address as nexthop in that case.

Thanks,
Vishwas

---

From: Sina Mirtorabi <[email-masked]>
Date: Sat, 30 Nov 2002 07:32:50 -0800

Vishwas,

Vishwas> >> why we cannot have global addresses as next hop.

Acee> > You are guarenteed to have a link local address on all OSPFv3
Acee> > interfaces while there is no guarentee that you have a
Acee> global address.
Acee> > I believe this is the rationale for using the link local.

Vishwas> I agree to the fact that all IPv6(not OSPFv3 in context of the
IPv4
Vishwas> discussion;-)) interfaces will have link-local addresses,
Vishwas> besides I am ok with the fact of using Link-local addresses
Vishwas> for nexthop. However I think saying that nexthops "will" be
Vishwas> link-local addresses would be wrong, we can have global
Vishwas> addresses in Link-LSA's and as nexthop(also the reason below)

having global address in Link-LSA is irrelevant to next hop calculation,
the fact of carrying global IPv6 address in Link-LSA is just related to
announcing all global Ipv6 address of a multi-access link to DR so that
it will announce them in its intra-area-prefix LSA

Link-local address is the only address to be always available ( as Acee
said ) therefore this address can always be used to reach your nexthop
and you should not need any other address even a node can have multiple
IPv6 address

Vishwas> >> Besides for some conditions
Vishwas> >> of redistribution we may require global scope addresses
Vishwas> for nexthop.

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

Sina

---

From: Yasuhiro Ohara <[email-masked]>
Date: Sun, 1 Dec 2002 03:40:09 +0900

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

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

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

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

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

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