Learn the key characteristics of Generic Routing Encapsulation (GRE) tunnels, including supported traffic types, encapsulation overhead, and TTL handling.
Table of Contents
Question
Which statement is correct about GRE tunnels?
A. The time-to-live field in the tunnel payload is decremented during transit.
B. GRE tunnels only support encapsulating non-IP traffic.
C. There are 20 bytes of overhead with GRE encapsulation.
D. GRE tunnels only support encapsulating IP traffic.
Answer
C. There are 20 bytes of overhead with GRE encapsulation.
Explanation
GRE (Generic Routing Encapsulation) is a tunneling protocol that encapsulates a wide variety of network layer protocols inside virtual point-to-point links or between two endpoints over an IP network.
Some key facts about GRE tunnels:
- GRE can encapsulate both IP and non-IP traffic (like IPX, AppleTalk, etc). So statements B and D are incorrect.
- The GRE header adds 4 bytes of overhead. Along with this, the outer IP header used for the GRE tunnel adds another 20 bytes (assuming IPv4). So in total, GRE encapsulation adds 24 bytes of overhead. Statement C is close but slightly off.
- The TTL (time-to-live) field is in the delivery header of the outer IP packet transporting the GRE tunnel, not in the GRE header itself or the original inner IP packet. The outer TTL gets decremented, but not the inner packet’s TTL. So statement A is incorrect.
In summary, GRE is a versatile tunneling protocol that supports multiple passenger protocols with an added overhead of 24 bytes per packet. The TTL handling described in the question does not apply to the GRE payload itself.
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