Understanding IPv6 next hops

[Neil Jerram]

On Thu, Feb 6, 2020 at 3:22 PM Ondrej Zajicek <santiago at crfreenet.org>
wrote:

> On Thu, Feb 06, 2020 at 12:34:00PM +0000, Neil Jerram wrote:
> > Good morning all!
> >
> > I'm debugging a situation where I'm seeing different IPv6 next hop
> > behaviour in two setups with different versions of my team's software.
> >
> > In both setups:
> > There are 3 routers A, B and C, all peered with another router X.
> > They are all on the same L2 bridge, and have global IPv6 addresses in the
> > 2001:20::/64 subnet.
> > A, B and C all export a route for fd00:10:96::/112
> > ...
> > Any ideas?  Can you advise where I should look or check next, to try to
> > understand why the UPDATE message has two next hop addresses in one
> setup,
> > but only one in the other?
>
> Hi
>
> Check code in IPv6 version of bgp_create_update(). It depends on how
> sender get the routes (local or received, were they received alredy with
> link-local next hop, were the next hop modified) and whether it is IBGP or
> EBGP and whether next hop is the same as sender.
>
> > Also, does the passing of two next hop addresses in setup #1 fully
> explain
> > why the ECMP routes programmed into the kernel use link-local gateway
> > addresses?
>
> Yes, link-local next hop is preferered as direct gateway.
>
> > Also, are the routes with global next hops more correct in some sense
> than
> > those with link-local next hops; or vice versa?  Would you expect them
> both
> > to forward data correctly?
>
> Well, it is a bit strange quirk of IPv6 BGP. In general, both global and
> link-local next hops should be sent when sender, receiver and global next
> hop are on the same subnet. Global next hop is used for recursive next
> hop evalulation, while link-local is used for forwarding.
>

Thank you very much Ondrej for all this.  I will work through understanding
and checking the details that you have provided.

Best wishes,
   Neil
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