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

[Sebastian Hahn]

Hi,

> On 6. Dec 2021, at 16:48, Ondrej Zajicek <santiago at crfreenet.org> wrote:
> On Mon, Dec 06, 2021 at 09:18:30AM +0100, Sebastian Hahn wrote:
>> 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?
> 
> 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.

OK, good to know you read it the same way.

> 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.

Thanks, I ended up doing it and it works like a charm with bird 2.0.8.

All the best
Sebastian