Junos supports not fragmenting jumbo frams on routed interfaces from 11.2.
This is the kb with the configuration of the mtu on the Routed vlan interface
http://kb.juniper.net/InfoCenter/index?page=content&id=KB22255
This it the description of the RVI in the documentation.
http://www.juniper.net/techpubs/en_US/junos10.4/topics/concept/bridging-ex-series-understanding.html?searchid=1336734033987
Routed VLAN Interface
In a traditional network, broadcast domains consist of either physical interfaces connected to a single switch or logical interfaces connected to one or more switches through VLAN configurations. Switches send traffic to hosts that are part of the same broadcast domain, but routers are needed to route traffic from one broadcast domain to another and to perform other Layer 3 functions such as traffic engineering. EX Series switches use a Layer 3 routed VLAN interface (RVI) named vlan to perform these routing functions, using it to route data to other Layer 3 interfaces. The RVI functions as a logical router, eliminating the need for having both a switch and a router.
The RVI (the vlan interface) must be configured as part of a broadcast domain or virtual private LAN service (VPLS) routing instance for Layer 3 traffic to be routed out of it. The RVI supports IPv4, IPv6, MPLS, and IS-IS traffic. At least one Layer 2 logical interface must be operational for the RVI to be operational. You must configure a broadcast domain or VPLS routing instance for the RVI just as you would configure a VLAN on the switch. Multicast data, broadcast data, or unicast data is switched between ports within the same RVI broadcast domain or VPLS routing instance. The RVI routes data that is destined for the switch’s media access control (MAC) address.
Jumbo frames of up to 9216 bytes are supported on an RVI. To route jumbo data packets on the RVI, you must configure the jumbo MTU size on the member physical interfaces of the RVI and not on the RVI itself (the vlan interface). However, for jumbo control packets—for example, to ping the RVI with a packet size of 6000 bytes or more—you must explicitly configure the jumbo MTU size on the interface named vlan (the RVI).