350-401 Question #208: Real Exam Question with Answer & Explanation

Question

Refer to the exhibit. R1 is able to ping the R3 fa0/1 interface. Why do the extended pings fail?

Options

• AThe maximum packet size accepted by the command is 1476 bytes.
• BR3 is missing a return route to 10.99.69.0/30
• CR2 and R3 do not have an OSPF adjacency
• DThe DF bit has been set

Explanation

Explanation

Setting the DF (Don't Fragment) bit in an extended ping tells routers along the path that the packet cannot be fragmented. When a packet exceeds the MTU of a link and the DF bit is set, the intermediate router must drop the packet and send an ICMP "Fragmentation Needed" message back to the source - this is why the larger packet sizes (1500 bytes) fail while a standard ping succeeds with smaller packets. Option B is incorrect because R1 can already ping R3's fa0/1, confirming a return route exists. Option C is wrong for the same reason - an existing successful ping proves OSPF adjacency and routing are functioning. Option A is incorrect because 1500 bytes is a standard, supported packet size; the issue is not a command limitation but rather the MTU ceiling combined with the DF bit preventing fragmentation.

Memory Tip: Think of the DF bit as a "Do Not Bend" sticker on a package - if it won't fit through a slot, the postal worker can't fold it and must return it. If pings work at small sizes but fail at larger sizes, always suspect MTU + DF bit before assuming routing problems.

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