[FROG] What causes a prefix received from a downstream to be "invalid"?

[Michael Hohl]

Hello FRRouting community,

I've tried to add an IPv6-only downstream to my dual-stack FRRouting-powered
BGP setup, but for some reason the prefix of that downstream wasn't distributed
to my upstream ISP. So, I consulted `show bgp summary` and spotted that this is
the only route which isn't marked as `valid` indicated by a `*`. RPKI isn't 
enabled, otherwise I would have immediately suspected that to cause some prefix
to be invalid. The only thing that makes this downstream different is that it
is IPv6-only.

What does it mean that a (received) prefix is considered "invalid"?

Here is the output for `show bgp 2001:XX:YY::`:

```
BGP routing table entry for 2001:XX:YY::/48
Paths: (1 available, no best path)
  Not advertised to any peer
  ZZZZZZ
    2a0d:f300:0:VVVV::1 (inaccessible) from 2a0d:f300:0:VVVV::1 (193.219.VV.VV)
      Origin IGP, invalid, external
      Last update: Mon Nov  9 09:00:00 2020
```

ZZZZZZ              ... is the downstreams ASN
2001:XX:YY::        ... is the IPv6 prefix the downstream wants to announce
2a0d:f300:0:VVVV::1 ... is the IPv6 address used to run the BGP router of the 
                        downstream
193.219.VV.VV       ... is the IPv4 address assigned by me to the downstream, 
                        but he doesn't really use that one

In case the FRRouting version is relevant for this issue, I'm running 7.3.1.

Thanks in advance for any help,
Michael