IP in IP Tunneling
IP in IP tunneling refers to the encapsulation of an IP packet as a payload in another IP packet. ASR9K routers support IP in IP tunnels with all possible combinations of IPv4 and IPv6; that is, IPv4 over IPv4, IPv6 over IPv4, IPv4 over IPv6, and IPv6 over IPv6. For example, an IPv4 over IPv6 refers to an IPv4 packet as a payload encapsulated within an IPv6 packet and routed across an IPv6 network to reach the destination IPv4 network, where it is decapsulated.
IP in IP tunneling does not require any additional header such as a GRE header used in the GRE tunnels. So, IP in IP tunneling is preferred over GRE tunnels if both the networks are IP networks.
The following are not supported in IP in IP tunnels:
Path MTU (Maximum Transmission Unit) discovery
DF (Do not Fragment) bit configuration in IPv6 tunnel mode.
If DF bit is configured for the tunnel interface, you cannot enable IPv6 tunnel mode.