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Exam 350-401 topic 1 question 81 discussion

Actual exam question from Cisco's 350-401

Question #: 81
Topic #: 1

Refer to the 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 is discarded on router B
B. The packet arrives on router C without fragmentation
C. The packet arrives on router C fragmented
D. The packet is discarded on router A

Show Suggested Answer

Suggested Answer: C 🗳️

by Chkoupipi2 at April 27, 2021, 9:52 a.m.

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Highly Voted 3 years, 6 months ago

GRE is 20 IP Header Bytes and 4 GRE Header Bytes and another 4 Bytes if a Tunnel key is used. 1500 + 28 > 1500 so the packet has to be fragmented to traverse the ethernet having a Maximum Transmission Unit of 1500 Bytes. If the DF bit was set, the packet would be discarded.

upvoted 41 times

HarwinderSekhon

1 year, 7 months ago

Max packet size that can transfer without fragmentation is 1476 (Default MTU) of GRE. Anything larger than 1476 will be fragmented or dropped (if Df-bit is set)

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Asymptote

2 years, 1 month ago

Excellent

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Most Recent 6 months, 2 weeks ago

Selected Answer: C

C is correct

upvoted 1 times

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flash007

1 year, 4 months ago

without DF set the packet will arrive fragmented

upvoted 1 times

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techriese

1 year, 5 months ago

Selected Answer: C

C is correct

upvoted 2 times

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0motor

1 year, 5 months ago

No MTU command needs to be run on the tunnel interface. When building a GRE tunnel the MTU on the tunnel interface is automatically decreased by 24 bytes to account for the overhead of the GRE header and new IP header. With physical interfaces set at 1500 there should be no fragmentation on a 1500 byte packet. https://community.cisco.com/t5/networking-knowledge-base/gre-tunnel-mtu-interface-mtu-and-fragmentation/ta-p/3673508

upvoted 2 times

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Pudu_vlad

2 years, 5 months ago

C is correct

upvoted 1 times

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ciscolessons

2 years, 9 months ago

Selected Answer: C

voting. provided answer is correct

upvoted 1 times

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[Removed]

2 years, 11 months ago

C is answer.

upvoted 1 times

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Ed394

3 years, 1 month ago

https://community.cisco.com/t5/networking-documents/gre-tunnel-mtu-interface-mtu-and-fragmentation/ta-p/3673508

upvoted 1 times

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error_909

3 years, 2 months ago

The given answer is correct.

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coysm

3 years, 7 months ago

I believe it's the extra overhead created by GRE encapsulation that will make the packet too large and cause fragmentation.

upvoted 2 times

AliMo123

3 years, 7 months ago

when df bit is cleared, fragmentation occurs, so the packet arrives fragmented.

upvoted 4 times

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Chkoupipi2

3 years, 7 months ago

I think fragmentation starts when the packet size exceeds 1500, here we have a value which exactly equals 1500, in such a case fragmentation will occur or not ?

upvoted 2 times

mumenkm

3 years, 3 months ago

MTU should be 1476 to avoid frag

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AliMo123

3 years, 6 months ago

remember, IP header takes 20 bits plus GRE4 bits. that will leave us with 1476 so MTU should be 1476 to avoid frag not 1500.

upvoted 3 times

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