Hi,
The version you are running is affected by this PR.
Moreover, the recommended versions have been updated recently, please go through the following link :-
http://kb.juniper.net/InfoCenter/index?page=content&id=KB21476&smlogin=true
This doesn not seem to be an issue with the MTU.
The PR has been resolved in the following and their subsequent releases :-
12.1X46-D45 12.3X48-D26 12.3X48-D30 15.1X49-D40
Regards,
Sahil Sharma
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