IP-IP tunnels are a type of tunnels that use IP as both the encapsulating and encapsulated protocol.

IP-IP tunnels are simple and easy to configure, but they do not provide any security or authentication

features. IP-IP tunnels only support encapsulating IP traffic, which means that the payload of the

inner packet must be an IP packet. IP-IP tunnels cannot encapsulate non-IP traffic, such as Ethernet

frames or MPLS labels1.

Option A is correct, because IP-IP tunnels only support encapsulating IP traffic. Option B is incorrect,

because IP-IP tunnels only support encapsulating non-IP traffic. Option C is incorrect, because the

TTL in the inner packet is not decremented during transit to the tunnel endpoint. The TTL in the outer

packet is decremented by each router along the path, but the TTL in the inner packet is preserved

until it reaches the tunnel endpoint2. Option D is incorrect, because there are 20 bytes of overhead

with IP-IP encapsulation. The overhead consists of the header of the outer packet, which has a fixed

size of 20 bytes for IPv43.

Reference:

1: IP-IP Tunneling 2: What is tunneling? | Tunneling in networking 3: IPv4 - Header