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shelley_wang
Beginner

IP fragmentation

                           G1/2                         G1/1

PC1 ---------- R1 ------------------------------------------- R2 --------------- PC2

                           MTU = 600 bytes

All interfaces have default interface MTU with length 1500 bytes except R1’s interface G1/2.

PC1 pings PC2 using cli “ping 100.1.1.2 size 1000”. So PC1 is sending the packet with IP header 20 bytes, and data 1000 bytes.

R1 has to fragment the packet before sending out interface G1/2.

Q1: The fragmented data length should be 600 bytes (MTU value) – 20 bytes (IP header) = 580 bytes. But 580 is not the multiple of 8 bytes, so the final fragmented data length should be 576 bytes.

Q2: The original 980 bytes data is fragmented into 2 pieces: 576 bytes, and 424 bytes

Destination PC2 assembles the 2 fragments.

Q3: PC2 then sends ICMP reply also with 2 fragments whose data sizes are: 576 bytes, and 424 bytes. Because if PC2 does not fragment, R1 will drop the packet which is bigger than R1’s interface MTU.

thx,

Jingyi

1 ACCEPTED SOLUTION

Accepted Solutions
Joseph W. Doherty
Hall of Fame Expert

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

Q3: PC2 then sends ICMP reply also with 2 fragments whose data sizes are: 576 bytes, and 424 bytes. Because if PC2 does not fragment, R1 will drop the packet which is bigger than R1’s interface MTU.

As already noted by Jcarvaja, PC2 would not reflect the fragmentation unless its NIC wasn't 1500.

If the R1<>R2 link MTU is only 600, then R2 should fragment just like R1.  However, if R1<>R2 is incorrectly configured, i.e. R1 believes MTU is 600 and R2 believes MTU is 1500, then R2 will send packets GT 600 to R1 which R1 will drop.

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2 REPLIES 2
Julio Carvajal
Advisor

Hello,

Exactly but I would say on point 3 that the PC will not fragment it,

I mean it will reply based on it's on NIC settings and then R2 will fragment packets it need it ( It will as by default the MTU for the link between the PC2 and R2 is of 1500  bytes,

Do you follow me

Regards

Julio Carvajal
Senior Network Security and Core Specialist
CCIE #42930, 2xCCNP, JNCIP-SEC
Joseph W. Doherty
Hall of Fame Expert

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

Q3: PC2 then sends ICMP reply also with 2 fragments whose data sizes are: 576 bytes, and 424 bytes. Because if PC2 does not fragment, R1 will drop the packet which is bigger than R1’s interface MTU.

As already noted by Jcarvaja, PC2 would not reflect the fragmentation unless its NIC wasn't 1500.

If the R1<>R2 link MTU is only 600, then R2 should fragment just like R1.  However, if R1<>R2 is incorrectly configured, i.e. R1 believes MTU is 600 and R2 believes MTU is 1500, then R2 will send packets GT 600 to R1 which R1 will drop.