full duplex and mtu question

[John DeDourek]

Jeffrey Ross wrote:
> 
> 
> John DeDourek wrote:
>>
>>>
>>> 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.
>>
> I don't fully agree (or disagree) however yes its true 10 and 100 Mb 
> ethernet use 1500 byte frames.  However I disagree to a point that for 
> Gigabit ethernet you should also run an MTU 1500.  Gigabit ethernet 
> allows for Jumbo frames or 9000 bytes (aka 9000 MTU) and there is 
> nothing wrong with using this MTU size.  In fact you will see a 
> performance gain...at least on paper as your system may not actually be 
> able to take advantage of the lower overhead.
> 
> Also there are plenty of other (dying) network interfaces  16Meg"broken" 
> ring=8136, ATM=4470, FDDI=4470 that have MTU's larger than the standard 
> 1500 any it may still be possible to take advantage of those links (on 
> internal networks)
> 
> 
> There are advantages to moving away from the 1500 MTU to jumbo frames, 
> *HOWEVER* you must make certain that all the devices on that segment all 
> share a common MTU.
> 
> Jeff
> 
Yes, I should have said 10/100 CSMA/CD Ethernets.  Is GigE CSMA/CD?
I think it is switched.  In any case, yes, my comment does not
apply to GigE.