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Re: full duplex and mtu question





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


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