350-401 Question #87: Real Exam Question with Answer & Explanation

Question

Which two GRE features are configured to prevent fragmentation? (Choose two.)

Options

• ATCP window size
• BTCP MSS
• CIP MTU
• DDF bit clear
• EMTU ignore
• FPMTUD

Explanation

GRE Fragmentation Prevention

TCP MSS (B) and Path MTU Discovery (PMTUD) (F) are the two GRE features specifically configured to prevent fragmentation. TCP MSS clamping adjusts the maximum segment size in TCP handshakes so that packets (including GRE overhead) fit within the tunnel MTU without needing to be fragmented. PMTUD dynamically discovers the smallest MTU along the path and signals endpoints to reduce packet sizes accordingly, preventing fragmentation before it occurs.

Why the distractors are wrong:

• TCP Window Size (A) controls flow control and throughput, not packet size or fragmentation.
• IP MTU (C) relates to MTU but is a configuration parameter, not a feature designed to prevent fragmentation - it defines the interface MTU rather than actively avoiding fragmentation.
• DF Bit Clear (D) does the opposite - clearing the DF (Don't Fragment) bit allows fragmentation rather than preventing it.
• MTU Ignore (E) bypasses MTU checks entirely, which can cause fragmentation problems rather than prevent them.

Memory Tip: Think "MSS and PMTUD = Prevent" - both are proactive solutions. MSS shrinks data before sending; PMTUD scouts the path first. Any option that allows or ignores fragmentation rules (D, E) is automatically wrong.

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