anyone familiar with setting mtu size? (1 Viewer)

diat150

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I changed my router and started having issues using rdp when I would vpn into our network. I found that mtu size may have been the culprit. I found the mtu size test and ran it and ended up at like 1464. well, now I start having issues loading pictures and stuff on forums. I run the mtu dos test again and it comes back with a lower number. I set my router and pc to the new lower number and run the test again and it comes back with an even lower number.

doesnt make much sense to me.
 

buzd

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What tests are running to determine MTU size?

MTU is the maximum size a device will allow a packet to be before it gets split. It doesn't mean it can't be smaller. You probably want most of your devices to have the same setting - it's definitely not the first thing I would mess with when troubleshooting.
 

Denzien

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Especially if you're on a wireless network: if your MTU is too big, you run a much greater chance of encountering a packet collision and having to request a re-send, slowing your transfer speeds down. If your MTU is too small, you will be burdened with additional overhead involved with sending more packets, slowing your transfer speeds down.

That's about all I remember. The more congested the network, the smaller the MTU should be and vice-versa.
 

zeetes

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1464 is a weird number to work with. 1500 is generally the baseline for most small networks (windows default is 1500). i concur with buzd, this is a setting that i normally wouldn't want to change unless i had to. now, if you are working with 10 gb/fcoe etc and using jumbo frames, then yeah, i'd see cause for concern.

you can ping from cmd line using "ping xxx.xxx.xxx.xxx -f -l 1500" to determine if the packets are going where you want them to without need of fragmentation. the xxx.xxx.xxx.xxx is obviously the IP/url, the -f forces full packet without fragmentation, and the -l is the mtu size. you can start with whatever size and keep subtracting from the number (generally go down by 10) until the reply is successful. then you add 24 (? maybe 28 i don't really remember :() to the number. the ping without fragmentation is a good indicator of the range you need to be in though.

there is some really good cisco documentation out there on it, i'll look to see if i can find it.
 

dgrant

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MTU for an Ethernet network should be 1500 bytes. The Ethernet header is 40 bytes, so if you account for that, the MTU is 1460 bytes. Sounds like it was configured properly already. Really it isn't a setting that many people need to adjust. If your router is sending across another network segment that isn't Ethernet, then the MTU will be different.
 

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