Setting jumbo protocol MTU in Juniper JUNOS
JUNOS has two forms of the "MTU" interface command: one at the
interface (layer-2) that affects all protocols generated by that
interface, and separate explicit commands for each layer-3
protocol specified in families. So two different ways of accomplishing
the Internet2-recommended 9000-byte IP (layer-3) MTU are:
The first example sets the MTU for all layer-2 frames, before subtracting
the bytes used by the layer-2 encapsulation (4 bytes for PPP or HDLC
encapsulation is assumed here). So that you can have an IP frame size of
9000 bytes, the interface MTU is set to 9004.
The second example separately sets the IPv4 and IPv6 MTUs to 9000 bytes,
showing how they could be different from each other.
NOTE: not all interfaces and versions of JUNOS support large MTUs. Check
with the Juniper JTAC if you're not sure. You may also find helpful this
Juniper document which lists MTU support by platform and interface type