BIGIP LTM 3400 and 3900

During our rollout of new hardware we did do a mixed pairing. There are certain options that you need to make sure dont exist. for eg if a b db provision.extramb variable is changed based on the 3900 ( say you allocate 2 GB to host) and sync this config, you will end up choking the 3400. 3600 vs 3400 was a stretch, but we managed to make it work for the transition (couple of months). 3900 vs 3400 might be stretching that further.