> The RFC says "ordinary IPv4 announcements are preferred to v4-via-v6 > announcements when the outgoing interface has an assigned IPv4 address", > but there is also the third option - using an IPv4 address from another > interface as a next hop.
The ways in which routing may fail are different. If you announce a v4-via-v6 route and the peer doesn't support v4-via-v6, then the v4-via-v6 announcements will be silently ignored, and the routed packets will either take a longer v4 route, or follow the default route. If, on the other hand, you announce an IPv4 address from a different interface, you might have grabbed an address from a management interface that's firewalled away from your transit network. The packets using that address as a next-hop are going to be silently dropped, which will cause a blackhole. A related point: Linux supports assigning the same IPv4 address to multiple interfaces, so if what you want is sharing a single IPv4 next-hop address between interfaces, you can already do that. -- Juliusz
