RFC 5549 - IPv4 Routes with IPv6 next-hop - Does it really exists?

Wed Jul 29 15:51:40 UTC 2020

> On Jul 29, 2020, at 02:13 , Saku Ytti <saku at ytti.fi> wrote:
> 
> On Wed, 29 Jul 2020 at 10:03, Vincent Bernat <bernat at luffy.cx> wrote:
> 
>> This is the solution Cumulus is advocating to its users, so I suppose
>> they have some real users behind that. Juniper also supports RFC 5549
>> but, from the documentation, the forwarding part is done using
>> lightweight tunnels.
> 
> I'm not sure if you claim otherwise, but no real 'tunneling' takes
> place, as far as I know, it's internal implementation detail having
> IPV6 next-hop for IPV4. I don't think there is any additional headers
> or any additional lookup or cost.
> Cisco supports extended nexthop encoding too, so it is fairly well
> supported by shipping products.

In reality, next hop isn’t really a layer 3 address. The layer 3 address is a stand-in that is resolved to
a layer 2 address for forwarding. The layer 3 next-hop address never makes it into the packet.
As such, the relationship between the destination address family and the next-hop address
family is mostly to avoid breaking the brains of humans. Software to handle mixed-address-families
in next hop vs. destination should be a relatively trivial difference from software that requires the
address families to match.

Owen