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 Brutsche
phil at brutsche dot us