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Changing the MTU on a DrayTEK 2820 or Cisco firewall
When using some devices with Sharedband you may need to lower the MTU on the WAN Port of your Firewall Device. xxxx is the MTU Value, this value needs to be 32 bytes lower than the Sharedband MTU, So with an MTU of 1492 on Sharedband this would need to be 1460.
Some networks require a different MTU so it is always best to check what your current MTU is in the Sharedband Customer Portal.Be careful changing things through telnet, you can stop your firewall from working. Its always best to backup the config before making any changes, that way if it goes wrong you may be able to factory reset it and restore the config. 

DrayTEK 2820 Series
To do this on a DrayTEK 2820 Series router you need to login through Telnet. Telnet is on by default on the LAN side of the draytek.

Once logged into the router it is WAN2 that the MTU needs to change on, to do this issue the following command:-
>wan mtu2 xxxx

Cisco (may vary by model/ios version)
To do this on some Cisco firewalls, you need to login through Telnet. Once logged into the router, issue the following commands (ignore the > character and substitute for the desired interface):-

>en
>show int
>conf 
>int
>ip mtu xxxx

Assuming you are satisfied with the result of the above, you may wish to save the running configuration to the startup configuration to make the change permanent.