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System unreachable after MTU change - HELP


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I have been investigating very poor upload speeds on my HP Mediasmart EX490 running DSM 06.2.   I decided to try changing MTU settings to see if that would help.  In fact, when the MTU was increased beyond 1500 I noticed a very significant increase in upload speed.


I should have stopped there, but to see what would happen with a further increase, I changed the MTU setting to 9000.   After the system rebooted I can no longer access it on the network.  I have tried the web interface as well as an SSH client, but I get no response in either case.


Is there some way for me to edit the USB boot disk to set the MTU back to a lower value so I can access the server?



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After doing a search on "xpenology MTU 9000" I found that there were a number of other folks who had stumbled into the same problem I did.   I found a suggestion to manually edit the synoinfo.conf file on the hard disk.   I found the bad setting of 9000 for eth0 MTU and I changed it back to 1500.  After rebooting the system I was able to reconnect to the server.


I am still puzzled by the dramatic change in upload speeds that I observed.   With an MTU setting of 1500 the upload speeds I was seeing with Speedtest.py was about 10Mbits/sec.  When I changed the MTU setting to 2000 the upload speed jumped to almost 80Mbits/sec.


Can anyone tell me why this dramatic change would happen, and what technique I should use to optimize my MTU setting?



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  • 4 weeks later...

I won't get into technical details but your MTU should be set to a value where packets don't get fragmented. Above that value your speed will take a hit. As for the MTU 9000 issue crashing the system it is due a driver. This is a known issue since the first loaders Jun released (v1.0). I beleive I was the first one to have had the problem. At least I think I was the first one to have reported it and thanks to this I was able to make a tutorial on how to access the system partition to modify the value back to 1500 or whatever value you want.


You can check what your MTU should be here: https://forum.peplink.com/t/how-to-determine-the-optimal-mtu-and-mss-size/7895

Edited by Polanskiman
Minor edits.
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