Refer to Exhibit. MTU has been configured on the underlying physical topology, and no MTU command has been configured on the tunnel interfaces. What happens when a 1500-bye IPv4 packet traverses the G

Question

Exhibits

Options

• AThe packet arrives on router C without fragmentation.
• BThe packet is discarded on router A
• CThe packet is discarded on router B
• DThe packet arrives on router C fragmented.

• A
  11% (3)
• B
  7% (2)
• C
  4% (1)
• D
  78% (21)

Explanation

GRE Tunnel Fragmentation Explained

Option D is correct because GRE encapsulation adds 24 bytes of overhead (20-byte outer IP header + 4-byte GRE header) to the original 1500-byte packet, making it 1524 bytes - exceeding the physical MTU of 1500 bytes. Since the DF (Don't Fragment) bit is cleared, the router is permitted to fragment the packet rather than discard it, so it arrives at Router C in fragments.

• Option A is wrong because the packet must be fragmented to traverse the physical links - it cannot pass through intact at 1524 bytes with a 1500-byte MTU.
• Option B is wrong because Router A will fragment (not discard) the oversized packet since the DF bit is cleared; discarding only occurs when DF is set.
• Option C is wrong for the same reason - no router along the path discards the packet when DF is cleared.

Memory Tip: Think of it as "DF cleared = Don't Fear fragmentation." When the DF bit is cleared, routers are allowed to fragment. GRE always adds overhead, so always expect fragmentation unless the tunnel or path MTU is adjusted to accommodate the extra bytes.

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