Hello Catalin,
the officially supported way would be:
15.1X53 --> 18.2 --> 18.4 --> 19.2 --> 19.4 --> 20.2
Since 15.1 every JUNOS release is considered as EEOL, 18.2 is EEOL, 18.3 is EEOL, 18.4 is EEOL too.
However, there is a good chance that even a direct upgrade from 15.1X53 to 20.2 can work, but this always depends on your config (20.3 or 20.4 is not released yet for EX3400).
As always, if you are running a mission critical network, copy your config to an EX3400 in your lab and check if an upgrade from 15.1X53 to 20.2 works.
Please note that it is not recommended to install the last and greatest release, as it is the most unstable release as well.
Juniper officially suggests Junos 18.2R3-S<latest> for EX3400, but newer releases eg. 18.4 or some 19 releases will work as well nowadays.
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Original Message:
Sent: 11-19-2020 10:41
From: Unknown User
Subject: Upgrade EX3400
Hello,
Sorry for posting again a thread about EX3400 switches upgrade path but I'm trying to have the smallest amount of jumps from version 15.1X53 to 20.4R2.
So, I'm wondering if I must do:
15.1X53 -> 17.1R1 -> 18.1R1 -> 19.1R1 -> 20.4R2
Thanks a lot for your feedback,
Catalin