What is the protocol number for Generic Routing Encapsulation (GRE)?

What is the protocol number for Generic Routing Encapsulation (GRE)?

• A. 46
• B. 47
• C. 48
• D. 49

Answer: (B)

The key idea is that the IP header uses a protocol field to identify what kind of payload is inside. GRE is signaled by the protocol number 47, so the packet is treated as a GRE-encapsulated frame that will be processed to decapsulate or route the inner payload. In practice, GRE wraps other network-layer protocols inside an IP tunnel, which is common in VPN setups. The other numbers simply correspond to different, non-GRE protocols, so they wouldn’t indicate GRE.

The key idea is that the IP header uses a protocol field to identify what kind of payload is inside. GRE is signaled by the protocol number 47, so the packet is treated as a GRE-encapsulated frame that will be processed to decapsulate or route the inner payload. In practice, GRE wraps other network-layer protocols inside an IP tunnel, which is common in VPN setups. The other numbers simply correspond to different, non-GRE protocols, so they wouldn’t indicate GRE.