full duplex and mtu question

[Jeffrey Ross]

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