SymptomsYour VMs/CTs are located in one private network, an IPv6 network.
If you set vzpriv_filter_host sysctl parameter to ON on the host, those VEs will not be able to see each other.
CauseHost acts as router between VEs on the node, and if its traffic is filtered in private network it cannot simply send “ndisk” packets to requestors. Thus VEs are not able to ping each other.
ResolutionThe case is studied by our development team.
If you want VEs to be able to see each other across a private IPv6 network – do not filter host traffic in that private network.
To verify that you are not doing it open /etc/sysctl.conf file
# vi /etc/sysctl.conf
Now add value (or simply change it if it's set to NO):
Close and save the changes. Type the following command to load sysctl settings from the file /etc/sysctl.conf file:
# sysctl -p
Read man page of sysctl for information:
$ man sysctl
Note: The above issue does not exist for IPv4 networks.