You will need to make sure you do the same on the SonicWall.
The setting you are looking for is:
VPN -> Advanced -> Enable Fragmented Packet Handling
Kristian Shaw wrote:
> If you have one of the later 1.2x versions there is an option in the
> Advanced System Config to allow fragmented packets from IPSEC connections.
> This will apply to inbound connections from the SonicWall.
> You also need to ensure that any outbound rules have the tickbox to allow
> fragmented packets too. This will apply to outbound connection from the
> m0n0wall to the SonicWall.
phil at brutsche dot us