OSPF/general question: Using v6 next-hop for v4 routes

[Ondrej Zajicek]

On Mon, Dec 06, 2021 at 09:18:30AM +0100, Sebastian Hahn wrote:
> Hi there,
> 
> I've been trying to remove v4 transit nets from my setup. This seems to
> work well for BGP (both eBGP and iBGP) - I have routes for all external
> v4 addresses with the appropriate next hop set. For my internal routing,
> I'm using OSPF, but it seems OSPF does not allow v6 next hops for v4
> routes. Checking RFC 5838 seems to imply that this is a general design
> choice for OSPF, but I am not sure if my reading is correct. Also, are
> there any practical drawbacks for this kind of setup that I may have
> missed when choosing it for BGP?

Hi

You are right that RFC 5838 does not allow v6 next hops for v4 routes.
I am not sure why, it could be easily done. Perhaps they just did not
expect universal support for that in forwarding planes in time when
RFC 5838 was written and they did not want to complicate negotiation
of capabilities.

But note that RFC 5838 explicitly allows nexthops outside of iface
network prefix. So in principle, you do not need v4 transit nets,
just one v4 host/loopback address per router. I am not sure that BIRD
implementation would work with that setup but if not, that could be
easily fixable.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
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"To err is human -- to blame it on a computer is even more so."