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Does MTU effect packets that are received on an interface ?

Unanswered Question
Jun 19th, 2012
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this might sound like a dumb question


R1 (1500) <-> (1400) R2 (1400)<->(1500) R3


with the above setup if R3 sends a ping packet to R1 with a packet size of 1500 and df-bit set, Will it be dropped by R2 at the ingress interface or at the egress interface towards to R1.


MTU meaning Maximum "Transimission" Unit I am assuming that the ping will be dropped at egress interface of R2 towards to R1, but am not 100 % sure about this

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Giuseppe Larosa Tue, 06/19/2012 - 04:52
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Hello Vikram,

in your scenario the packet would be discarded at the ingress interface of R2, as it would be too big to be treated/received.

A packet too big is classified as an input error of type giant on a LAN interface and these counters are shown in show interface output.


MTU value should match at both ends of a single link.


Hope to help

Giuseppe

vijay.swaminathan Tue, 06/19/2012 - 06:24
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Please allow me to join this thread since I too had the same questions :


What would happen if we DF bit it not set?


Typically on a larger network it is practically impossible to know the MTU size of each of the router in the path? so how do we make the communication possible ? or is it mandatory to know the MTU size of all the interfaces on a flow from src to dstn?


Please advice.

Thanks

-Vijay

Giuseppe Larosa Tue, 06/19/2012 - 06:43
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Hello Vijay,

in the scenario described by the original poster with both R2 interfaces MTU set to 1400, the DF bit settings on the received packet has no influence on the packet fate: it will be discarded and giant input error counter is incremented by 1.


The 1500 byte IP packet would be simply too big to be processed. If the ingress interface has MTU 1500 in that case the DF settings play a role allowing packet fragmentation if not set.


There are some mechanisms like path mtu discovery that allow to discover the MTU on the path from source to destination.

This path MTU discovery is not always possible and it has to be supported by endpoints

see

http://www.cisco.com/en/US/docs/ios/12_1/iproute/configuration/guide/1cdip.html#wp1001001


The general trend is to avoid fragmentation as much as possible:

on service provider network we see high MTU values on backbone and edge links to accomodate all possible services, Values as high as more then 9000 bytes are becoming common.


On VPN services built on internet access the trend is to use a reduced MTU to avoid fragmentation to take in account all the overhead


in these cases from 8 bytes overhead of PPPoE ( requires IP mtu 1492)  to IP MTU values as low as 1300 bytes for some IPSEC+GRE scenarios



Hope to help

Giuseppe

Sandeep Choudhary Tue, 06/19/2012 - 06:45
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HI Vikram,


I am totaly agree with Giuseppe.


MTU problems may result in degraded network service, but may not affect some users' abilities to access the required applications, so sometimes MTU problems go unreported. Other times, MTU problems cause severe lags in network logon times,  and cause to stop functioning entirely. 

The MTU must be set to the same setting on all your router interface.

if not same then ......still it will work but ..later on you will face the problems.


Regards

Please rate if it helps.

Sandeep Choudhary Tue, 06/19/2012 - 06:58
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Hi Vijay,


1.What would happen if we DF bit it not set?

If a packet is received which has the Don't Fragment (DF) bit set in the packet header, the packet is not fragmented, but instead discarded and if DF is not set then it Fragmented the packet according to the interface MTU size of the next router.


2.I also do not know much about,how to find the MTU of the all router in network because it differs according to the diff routers models. .


1500The biggest-sized IP packet that can normally traverse the Internet without getting fragmented. Typical MTU for non-PPPoE, non-VPN connections.
1492The maximum MTU recommended for Internet PPPoE implementations.
1472The maximum ping data payload before fragmentation errors are received on non-PPPoE, non-VPN connections.
1460TCP Data size (MSS) when MTU is 1500 and not using PPPoE.
1464The maximum ping data payload before fragmentation errors are received when using a PPPoE-connected machine.
1452TCP Data size (MSS) when MTU is 1492 and using PPPoE.
576Typically recommended as the MTU for dial-up type applications, leaving 536 bytes of TCP data.
48The sum of IP, TCP and PPPoE headers.
The sum of IP and TCP headers.
28The sum of IP and ICMP headers.


Regards

Please rate if it helps

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