Exam JN0-351 topic 1 question 93 discussion

BCD are wrong A evenif paylod is not decremented in tunnel, it is while entering

they do not ask for header where GRE adds 4 + to 20 IP header.... they asked about overhead which 24 and for IPtoIP is 20

GRE encapsulation adds: 20 bytes for the outer IP header, and 4 bytes for the GRE header, bringing the total overhead to 24 bytes for a standard GRE tunnel. However, the GRE header itself is 4 bytes, but often "20 bytes" is referenced when focusing solely on the IP header without options or extensions. This item may be valid depending on interpretation, but the full GRE overhead is typically 24 bytes. Given the question phrasing, this is the most correct of the listed options.

The inner packet (also known as the payload packet) is not modified, except the time-to-live (TTL) field, which is decremented. The TTL field must be decremented to ensure that the packet does not live forever.

A. The time-to-live field in the tunnel payload is decremented during transit: This statement is correct. The TTL of the inner IP packet (tunnel payload) is decremented when traversing the GRE tunnel because GRE behaves like a standard routed interface. Each hop processes the packet, so the TTL is decremented to avoid loops in the network. C. There are 20 bytes of overhead with GRE encapsulation: Incorrect. GRE encapsulation adds 24 bytes of overhead: 20 bytes for the new IP header (IPv4 or IPv6). 4 bytes for the GRE header.

A is correct. The inner packet, also known as the payload, is not modified except time to live (TTL) which is decremented. TTL filed must be decrementd to ensure that the packet does not live forever.

C is Correct

Explanation: GRE (Generic Routing Encapsulation) adds a fixed overhead of 24 bytes, which includes a 4-byte GRE header and a 20-byte IP header. Thus, while C is somewhat misleading in its specific wording, it is generally accepted that the total overhead when encapsulating an IP packet is 24 bytes. Other Options: A: The TTL field in the GRE tunnel payload is not decremented by routers. B and D: GRE tunnels support encapsulating both IP and non-IP traffic.

A. The time-to-live field in the tunnel payload is decremented during transit. Explanation: GRE (Generic Routing Encapsulation) is a protocol that can encapsulate a wide variety of network layer protocols inside virtual point-to-point connections. When traffic is sent through a GRE tunnel, the TTL (Time to Live) of the original IP packet inside the GRE payload is decremented as the packet traverses the tunnel.