full duplex and mtu question
John DeDourek
dedourek at unb.ca
Mon Jul 2 23:10:35 UTC 2007
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.
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