Since I upgraded my servers to centos 6.5, all the new GRE tunnel will not work. After digging, I found the output of "ip add" is as follows: Code: 4: gre0: <NOARP> mtu 1476 qdisc noop state DOWN link/gre 126.96.36.199 brd 188.8.131.52 (note they are fake IPs) 5: gretap0: <BROADCAST,MULTICAST> mtu 1476 qdisc noop state DOWN qlen 1000 link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff For my other servers who are running centos 6.4 and has GRE tunnel working, the output of "ip add" will be Code: 5: gre0: <NOARP> mtu 1476 qdisc noop link/gre 0.0.0.0 brd 0.0.0.0 Compare the outputs and we can clearly see the differences. Is the failure of GRE tunnel due to these differences? I didn't create gretap, how come gretap? Thanks in advance!