ICMP and discard oversize frame

[Eric Boudrand]

Hello,

I am running a FreeBSD router with two ethernet cards. I have reduced
the MTU to 800 in order to generate ICMP packet "Fragmentation needed
but DF was set". I can find in log :

rl1: discard oversize frame (ether type 800 flag 3 len 875 > max 814)

But there is no ICMP sent. By default, is this feature disabled ? How 
can I see these ICMP packets generated ?

Regards.

Eric