Could it be that this MTU isn't allowed by the network device used? Some have artificial limits imposed by the drivers.
Can you successfully change the MTU value from the command line? Try: ip link set dev eth0 mtu 9000 What does that command return? ** Changed in: network-manager (Ubuntu) Status: New => Incomplete ** Changed in: network-manager (Ubuntu) Importance: Undecided => Medium -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1061304 Title: Configured ethernet MTU not set with IPv4 disabled To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1061304/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs