Re: full duplex and mtu question

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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.


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.

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