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| Mar 14 2013, 3:35 PM |
To determine the optimum MTU for Sharedband routers and if applicable, the 3rd party/outward facing routers that are connected to their WAN interfaces
The appropriate MTU that ADSL routers should be configured to operate with is normally provided by your ISP. This value is always equal to or less than 1492. Where you are provided different MTUs across multiple ISPs please ensure that your routers are all configured with the lowest MTU value given.
Should you encounter difficulty aquiring the MTU from your ISP or wish to test it yourself, guidance is available below.
If present, log into each of the outward facing/ISP routers and set the MTU value to be 1492.
Log into each of the Sharedband routers and set the MTU value to be 1492.
Start a command prompt. Go to Start and select Run. In the box, type in cmd (Windows Vista/7), and then hit enter.
In the cmd window type: ping -f -l 1460 google.co.uk
(the l is a lower case L, and note there are spaces after ping, f, l, and 1460)
You may get output that says something like "packet needs to fragment but DF set", or 100% packet loss, or “request timed out”.
If you do, then type that command again, reducing the value of 1460 by small increments until you find the largest number which works with no lost packets. Then just add 32 to this number and set it as the MTU in all of your routers (both 3rd party/outward facing and Sharedband) settings.
EG: max value with no packet loss = 1460, add 32 = optimum MTU of 1492.
It is important to understand that where a hardware firewall is present, this must be set to use the MTU 32 bytes lower (e.g. 1460) than that set on the Sharedband routers and this is further explained at http://support.sharedband.com/kb/kb8
The appropriate MTU that ADSL routers should be configured to operate with is normally provided by your ISP. This value is always equal to or less than 1492. Where you are provided different MTUs across multiple ISPs please ensure that your routers are all configured with the lowest MTU value given.
Should you encounter difficulty aquiring the MTU from your ISP or wish to test it yourself, guidance is available below.
If present, log into each of the outward facing/ISP routers and set the MTU value to be 1492.
Log into each of the Sharedband routers and set the MTU value to be 1492.
Start a command prompt. Go to Start and select Run. In the box, type in cmd (Windows Vista/7), and then hit enter.
In the cmd window type: ping -f -l 1460 google.co.uk
(the l is a lower case L, and note there are spaces after ping, f, l, and 1460)
You may get output that says something like "packet needs to fragment but DF set", or 100% packet loss, or “request timed out”.
If you do, then type that command again, reducing the value of 1460 by small increments until you find the largest number which works with no lost packets. Then just add 32 to this number and set it as the MTU in all of your routers (both 3rd party/outward facing and Sharedband) settings.
EG: max value with no packet loss = 1460, add 32 = optimum MTU of 1492.
It is important to understand that where a hardware firewall is present, this must be set to use the MTU 32 bytes lower (e.g. 1460) than that set on the Sharedband routers and this is further explained at http://support.sharedband.com/kb/kb8