full duplex and mtu question

[John DeDourek]

Jeffrey Ross wrote:
> 
> 
> Michael H. Semcheski wrote:
>>> chloe] Do you know whether there is any network issue if we don't 
>>> change MTU
>>> as same as our provider? eg: our upstream provider router is using 
>>> mtu 1600
>>> but our end is 1500.
>>
>> If your MTU is lower than your upstream, you'll end up sending a few
>> more packets than you need to.  (for 1500 vs 1600, about 1.066 times
>> to many)
>>
>> If your MTU is higher than your upstream, then you'll fragment your
>> packets.  When you try to send a single packet, it will get split into
>> two packets.  (if you have 1700 vs 1600, the 1700 will be split into a
>> 1600 and a 100 byte packet.)  That doubles the amount of packet
>> overhead you have to transfer, and doubles the number of packets that
>> have to be received.
>>
>> I'm not saying this authoritatively, but that's how I understand 
>> things to work.
>>
> 
> Just be careful that you have all the machines on the same network 
> (physical network/VLAN) all set to the same MTU otherwise bad things 
> happen.   MTU sizes can change once you pass through a router.  The 
> router will either fragment the packet providing the DF bit is not set 
> or send back an ICMP message stating that the MTU size has been exceeded 
> along with the correct/new MTU size to use.
> 
> Jeff
> 
Also note that some LAN's have a maximum MTU specified as part
of the physical design, and may not work (or work but with
poor performance) with a larger MTU.  Traditional Ethernet
(the kind called CSMA/CD had a maximum MTU of 1500.  Although
modern Ethernets have moved away from CSMA/CD, if that's an
Ethernet interface, I would advise leaving it at 1500.