ipv6 local net ping6 tests

gistfile1.txt

Problem: machines on internal can't ping machines on external (and vice versa), 
but only if the upstream connection is dead, if it is up everything works fine.

Prefix assigned 2001:41f0:61::/48
Public subnet   2001:41f0:61:::/64
Intern subnet   2001:41f0:61:1::/64

internal1: 2001:41f0:61:1:f2de:f1ff:fe45:6006/64
router   : 2001:41f0:61:1::1  (internal)
           2001:41f0:61::2    (external)
external1: 2001:41f0:61::6

Tests:
ping6 internal1 -> router2 (internal interface) OK
ping6 internal1 -> router2 (external interface) OK (forwarding )
ping6 router    -> internal1 OK
ping6 router    -> external1 OK
ping6 external1 -> router OK

ping6 internal1 -> external1 FAIL
ping6 external1 -> internal1 FAIL

TcpDump ping6 results on 'external1':
- echo request received
- echo reply sent ('produced' ?)

TcpDump ping6 results on 'router':
- echo request from 'internal1' seen
- echo reply from 'external1' NOT seen  <--- Nothing in ip6tables log on both machines???


on internal1: 
>ip route get 2001:41f0:61::6
2001:41f0:61::6 from :: via fe80::213:72ff:fe66:fe62 dev enp0s25  src 2001:41f0:61:1:f2de:f1ff:fe45:6006  metric 0 
    cache  hoplimit 64
    
on external1:
> ip route get 2001:41f0:61:1:f2de:f1ff:fe45:6006
2001:41f0:61:1:f2de:f1ff:fe45:6006 from :: via 2001:41f0:61::1 dev eth0  src 2001:41f0:61::6  metric 0 
    cache