Jumbo frames on PE 1950

[Matt Lawrence]

I'm running CentOS 5.2 with a much more recent kernel on a bunch of PE 
1950 systems.  I have set the maximum MTU on the switch to 9000 and I'm 
trying to make jumbo frames work.  No luck so far, with any MTU greater 
than 1500,  netperf just gets the error: netperf: receive_response: no 
response received. errno 110 counter 0

Any idea what I have missed?

-- Matt