more looking for a best practice here
do I need to:
1. set the MTU on the phsical ports or
2. set the MTU on the IRB or
3 both ?
on cisco i was able to just set the SVI and it was good to go
From my experience it works like this:
root@labdc01-leaf01# show |compare
[edit interfaces irb]
+ mtu 2500;
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.
Note: Tested on QFX5110 running 17.4R2 but expect exact same behaviour on QFX5200.