When you are implementing GRE tunnels, why might you experience MTU and IP fragmentation- related issues?

Question

Options

• AGRE adds an extra 22 bytes.
• BGRE adds a 2-byte GRE header and a new 22-byte IP header.
• CGRE adds an extra 4-byte GRE header and a new 18-byte IP header.
• DGRE adds an extra 4-byte GRE header and a new 20-byte IP header.

• A
  2% (1)
• B
  2% (1)
• C
  6% (3)
• D
  91% (48)

Explanation

GRE encapsulation wraps the original (inner) IP packet with two additional headers: a 4-byte GRE header and a new 20-byte outer IP header (assuming no IP options). This adds 24 bytes of overhead to every packet. If the original packet is already at or near the path MTU (typically 1500 bytes on Ethernet), the encapsulated packet will exceed the MTU of the outgoing interface. This causes either fragmentation of the outer packet (degrading performance) or packet drops if the DF (Don't Fragment) bit is set on the inner packet. Solutions include reducing the tunnel interface MTU, enabling tunnel path MTU discovery, or using TCP MSS clamping.

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