Configuring MTU network interface for internet provider via Cable modem

Question

Hi, 
 Every morning, i need to wake up my WAN interface by pressing the Renew button for assigning a new ip address (remark: it keeps the same ip address for a long period) or i'm not able to connect on Internet. 
 Somewhere i've read that the problem can be a bad MTU value assigned to the network interface. Normally the MTU for my cable modem should be 1400. 
 When i change this value from 576 to 1400 manually then after a short moment the value is set back automatically on 576. 
 Any idea why this value returns to its original state and why i can't force it manually? 
 
 Thank you for any suggestion :) 
 Fred

zaphod · Accepted Answer

you run into a known bug: 
 
 https://community.sophos.com/products/unified-threat-management/f/hardware-installation-up2date-licensing/80641/sophos-utm-9-407-3-released 
 
 read that post... there are some description in it how you can manualy fix the mtu value... 576 is almost wrong for any cable providers..

sachingurung · Answer

Hi All, 
 The steps can be referred from the links here: 
 https://community.sophos.com/products/unified-threat-management/f/hardware-installation-up2date-licensing/80641/sophos-utm-9-407-3-released/306734#306734 
 https://community.sophos.com/products/unified-threat-management/f/hardware-installation-up2date-licensing/80641/sophos-utm-9-407-3-released/307038#307038 
 Hope that helps.