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

Wed Jul 29 18:10:46 UTC 2020

> On Jul 29, 2020, at 09:43 , Douglas Fischer <fischerdouglas at gmail.com> wrote:
> 
> Does anybody here knows what Gambiarra means?

The english translation would be “Jury Rig” or “Hack”.
Synonyms include “McGyverism”, “Rube Goldberg”, “Kludge”, etc.

Foreign address family as next-hop is definitely in this category.

> 
> Alejandro mentioned that IPv6 NextHop on IPv4 routing breaks traceroute and difficult troubleshooting.

It doesn’t really break trace route, but it does complicate troubleshooting.

The next hop device won’t know that the IPv4 packet arrived via IPv6 next hop. If the device has an IPv4 address, it will still
report that in the trace route. Of course, that won’t match the expected next-hop from the routing table on the previous device,
but it will still be reported.

If it doesn’t have an IPv4 address, then one has to wonder how that’s going to work for what it will do with the packet anyway.
In such a case, I would expect that it breaks more than trace route.

Troubleshooting is difficult because it requires significant indirection to figure out what’s really going on and because it creates
a good bit of cognitive dissonance in the human analysis part of the troubleshooting effort.

> Well... Since a while I have been thinking about a Gambiarra that I'm using on other scenarios, but I think could help to reduce de bad impacts of IPv6 NextHop on IPv4 routing.
> 
> O router with several interfaces with IPv6 only and at least one public IPv4 /32 on his loopback.
> On the IPv4 address on each of that v6 only interfaces, use "IP address unnumbered loopback 0".
> 
> This would make the ICMP responses for TTL expired be sourced with that public IPv4.
> 
> Would not be as good as one public IP for each interface, but at least, on a traceroute, would be possible to Defined what ASN is responsible for that hop, and exactly in what router it occurs.

You most likely get the same result whether you add the unnumbered configuration or not on a router where the only IPv4 address is on the loopback interface.

Owen