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-byte IPv4 packet traverses the GRE tunnel from host X to host Y, assuming the DF bit is cleared?
- A. The packet arrives on router C without fragmentation.
- B. The packet is discarded on router A
- C. The packet is discarded on router B
- D. The packet arrives on router C fragmented.
Explanation: Like any protocol, using GRE adds a few bytes to the size of data packets. This must be factored into the MSS and MTU settings for packets. If the MTU is 1,500 bytes and the MSS is 1,460 bytes (to account for the size of the necessary IP and TCP headers), the addition of GRE 24-byte headers will cause the packets to exceed the MTU:
1,460 bytes [payload] + 20 bytes [TCP header]+ 20 bytes [IP header] + 24 bytes [GRE header + IP header] = 1,524 bytes
As a result, the packets will be fragmented. Fragmentation slows down packet delivery times and increases how much compute power is used, because packets that exceed the MTU must be broken down and then reassembled.
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