Does Different MTU Communicate

Does Different MTU Communicate

A dicey sitution occurs when MTU on different interfaces come in picture. Every is having a thought process that the lowest MTU on interface will negociate each other but in fact, it only works in case of path mtu is enabled. Assume a scenario where a R1 router is connected with R2 router. R1 interface is having MTU of 1500 bytes and latter is having MTU of 1600 bytes. If R1 sends a packets of 1501 bytes to R2 without DF bit set, R1 will fragment the packet and forwards it to R2 reassembles the packet and will forward a single packet of 1501 bytes to R1. R1 interface will drop the packet because the interface MTU is set to 1500 bytes and communiaction will never occur in case of packet size more than 1500 bytes.


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shivlu jain
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How to set DF bit from Windows Vista

In the last post we have seen how to check the mtu on windows vista. In this post we will see how to set the DF big on a packet.

On command prompt ping yahoo.com -f -l 1460; you will see the reply. Now ping yahoo with 1500 bytes, you receive packets need to be fragmented. -f is used for donot fragment.

Pinging yahoo.com [206.190.60.37] with 1500 bytes of data:

Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

regards
shivlu jain
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How to Check MTU on Windows Vista

MTU is one of the problems which is being faced by everyone when the media is ethernet. But in this post I am not going to tell you about the problems faced with MTU. Simply I wanna show a very simple command in vista which can help you to check the mtu size of your windows.

C:\>netsh interface ipv4 show interfaces

Idx Met MTU State Name
--- --- ----- ----------- -------------------
1 4275 4294967295 connected Loopback Pseudo-Interface 1
9 4230 1500 disconnected Wireless Network Connection 2
21 50 1500 connected Dial-up Connection
8 4230 1500 disconnected Local Area Connection


regards
shivlu jain
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