ipv6 disconnect from time to time

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http://forums.fedoraforum.org – hi guys, I have a problem about ipv6 on F17. My network support ipv6. I add Code: NETWORKING_IPV6=yes to /etc/sysconfig/network. I use Network-Manager to access network. After I connect the system to network, I can go to ipv6 sites and use transmission to download via ipv6. But 20-30 minutes later, I can't use the ipv6(but ipv4 is fine), including downloading and viewing webpage via ipv6. 'ping6 ipv6.google.com' shows 'connect: Network is unreachable'. Then I have to reconnect my network to use ipv6. Any one have suggestions about my ipv6 disconnect from time to time. Tha (HowTos)