thank youforyouranswers.
Ashvin: i keep in memory the pathfinder 
( very usefull link )
Rccpgm : My EX4200 are well with this system. You havefindthereasonwhy i want to update them : the end of support date. I prefer to have an system who have an support beccause if i call support they can help me.
So my question is, does Juniper will maintain the 12 branch or itis necessaryto passtoversion15 ???