Controls if fallback tunnels (like tunl0, gre0, gretap0, erspan0, sit0, ip6tnl0, ip6gre0) are automatically created. There are 3 possibilities (a) value = 0; respective fallback tunnels are created when module is loaded in every net namespaces (backward compatible behavior). (b) value = 1; [kcmd value: initns] respective fallback tunnels are created only in init net namespace and every other net namespace will not have them. (c) value = 2; [kcmd value: none] fallback tunnels are not created when a module is loaded in any of the net namespace. Setting value to “2” is pointless after boot if these modules are built-in, so there is a kernel command-line option that can change this default. Please refer to Documentation/admin-guide/kernel-parameters.txt for additional details.
Not creating fallback tunnels gives control to userspace to create whatever is needed only and avoid creating devices which are redundant.
Default : 0 (for compatibility reasons)