From my experience it works like this:
- if all interfaces where an vlan is used are using jumbo frames, the irb will have matching MTU.
- if interfaces have different mtu (jumbo on one and standard 1500 on another), the irb will inherit the lowest MTU value present on the switch.
- Configuring MTU on 'irb' (not irb.<unit>) will not be honored. Trying to set a MTU higher than your interface will even give you a commit warning:
root@labdc01-leaf01# show |compare
[edit interfaces irb]
+ mtu 2500;
{master:0}[edit]
root@labdc01-leaf01# commit check
warning: irb.10 ifd's config mtu 2500 is bigger than member link xe-0/0/0.0 ifd's mtu 1514 for family inet
So basically - for irb's you should handle it on the physical interfaces.
Examples:
- et-0/0/48 and xe-0/0/0 have MTU 9216. irb will then have MTU 9206.
- et-0/0/48 have MTU 9216 and xe-0/0/0 is default 1514. irb will then have MTU 1500.
Note: Tested on QFX5110 running 17.4R2 but expect exact same behaviour on QFX5200.