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MTU?

Hey guys,

I need a sanity check.

I have a couple of on going issues - larger downloads random fail and a Cloud based app randomly crashing for no reason.

So I did a ping <IP Address> -f -l 1480 and found it was Fragmenting.

I went down to 1444 and it stopped Fragmenting. So in the XG Network WAN interface changed the MTU to 1444 and MSS 48 less. Job done right..... Not so much

Today I check and the MTU is no longer working at 1444 bit now 1404. So tonight I dropped it to 1404. Gave it 15 minutes and did the exercise again and MTU now 1364

What gives?

If I change it to suit the MTU on the ping test it just lowers by 40 each time I test.

Am I missing something?

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0 Keyur over 6 years ago

Hi M8ey

MTU size is defined by the lowest MTU size in the path.

I would request you contact technical support and investigate the issue further. To open a service request, please visit- https://secure2.sophos.com/en-us/support/open-a-support-case.aspx

https://packetlife.net/blog/2008/aug/18/path-mtu-discovery/
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0 M8ey over 6 years ago in reply to Keyur

Keyur said:
I would request you contact technical support

Yeah there was already a ticket open for a Downloading issue which lead to MTU which lead to this.

Today I tried it on my other XG230 at another site.

Same thing happens!

Keep dropping the MTU in the UI and the test will fail until you drop it by another 40 bytes.

I am thinking XG is reading something other than it should.....

Tickets open if you want to look - 9128575

Anyone else tried this?

Ping 8.8.8.8 -f -l 1500 see what you get the MTU to - change it in XG - give it a while and try again - see if you can still ping that new MTU figure.

I was at 1480 - then it said 1444 so dropped to 1444 then tried and it said 1404 so dropped to 1404 waited then it suggested 1376 :-(
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0 Keyur over 6 years ago in reply to M8ey

Hi M8ey

Thank you for providing the service request number, I have asked the support engineer to contact you at earliest to troubleshoot the issue further.
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0 Keyur over 6 years ago in reply to M8ey

Hi M8ey

Thank you for providing the service request number, I have asked the support engineer to contact you at earliest to troubleshoot the issue further.
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