1—IPv4 GRE tunnel mode.
2—IPv6 GRE tunnel mode.
3—IPv4 over IPv4 tunnel mode.
4—Manual IPv6 over IPv4 tunnel mode.
5—Automatic IPv6 over IPv4 tunnel mode.
6—IPv6 over IPv4 6to4 tunnel mode.
7—IPv6 over IPv4 ISATAP tunnel mode.
8—IPv6 over IPv6 or IPv4 tunnel mode.
9—UDP-encapsulated IPv4 DVPN tunnel mode.
10—GRE-encapsulated IPv4 DVPN tunnel mode.
11—UDP-encapsulated IPv6 DVPN tunnel mode.
12—GRE-encapsulated IPv6 DVPN tunnel mode.
13—CRLSP tunnel mode.
14—IPv4 multicast GRE tunnel mode.
15—IPv6 multicast GRE tunnel mode.
16—IPv4 IPsec tunnel mode.
17—IPv6 IPsec tunnel mode.
22—DS-Lite AFTR tunnel mode.
23—CRLSP automatic tunnel mode.
24—UDP-encapsulated IPv4 VXLAN tunnel mode.
25—UDP-encapsulated IPv6 VXLAN tunnel mode.
You must specify the tunnel mode when creating a tunnel. After the tunnel is created, you cannot change the tunnel mode.