Exam JN0-351 topic 1 question 10 discussion

A. IP-IP tunnels only support encapsulating IP traffic. IP-IP tunnels are designed specifically to encapsulate IP packets within IP packets, enabling the transport of IP traffic over an IP network. They do not support encapsulating non-IP traffic. C is not correct because it suggests that the Time To Live (TTL) value in the inner IP packet would be decremented while the packet is in transit through the tunnel. When using IP-IP tunnels, the inner IP packet is encapsulated within an outer IP packet. The outer packet handles the delivery across the tunnel. The TTL in the inner packet is not decremented because the packet is effectively shielded from intermediate hops by the tunnel. Only the TTL of the outer encapsulating packet is decremented at each hop. The TTL field is used to prevent packets from looping indefinitely in a network by reducing the TTL by 1 at each hop. Once the outer packet reaches the tunnel endpoint, the inner packet is decapsulated, and its original TTL value is preserved.

In IP-to-IP as with GRE the inner packet TTL is not decremented. The encapsulating packet TTL is just like any other IP packet. The most correct answer is A.

A is correct

Correct: A

Answer C isn't correct, because it suggests that TRANSIT ROUTERS decrement TTL of the inner packet. Think about it - transit routers would need to be aware of the tunneling, to modify inner IP packet fields - the point of tunneling protocols is to make the inner packet transparent for the transit/underlay network. From Juniper Learning Portal: "The ENCAPSULATOR does not change the inner IP header, except to decrement the TTL and it remains unchanged during its deliver to the endpoint" - only router that encapsulates the inner packet decrements its TTL - during transit, the inner packet is unchanged. Because of that, the correct answer is A - as the name suggests, IP-IP tunneling isn't protocol-agnostic - it can only tunnel IP traffic.

Answer C isn't correct, because it suggests that TRANSIT ROUTERS decrement TTL of the inner packet. Think about it - transit routers would need to be aware of the tunneling, to modify inner IP packet fields - the point of tunneling protocols is to make the inner packet transparent for the transit/underlay network. From Juniper Learning Portal: "The ENCAPSULATOR does not change the inner IP header, except to decrement the TTL and it remains unchanged during its deliver to the endpoint" - only router that encapsulates the inner packet decrements its TTL - during transit, the inner packet is unchanged. Because of that, the correct answer is A - as the name suggests, IP-IP tunneling isn't protocol-agnostic - it can only tunnel IP traffic.

C. The TTL in the inner packet is decremented during transit to the tunnel endpoint. IP-IP tunnels encapsulate IP packets within other IP packets, allowing them to traverse networks that wouldn't normally support the original IP packets. This encapsulation involves the TTL (Time-to-Live) of the inner packet being decremented as it traverses through the tunnel towards its endpoint.

Overhead is 20 bytes, not 24 as GRE