Currently, SRX5400 (SRX5k RE-1800X4) is running with 15.1X49-D110.4 built 2017-09-08.
I have looked at the latest recommended version at https://kb.juniper.net/InfoCenter/index?page=content&id=kb21476
It says - 18.4R3-S2 service release is recommended for SRX5400 (SRX5k RE-1800X4).
Is it okay to upgrade directly from 15.1X49-D110.4 to 18.4R3-S2 for SRX5400 (SRX5k RE-1800X4)?
I don't think you will be able to perform a direct ugrade from 15.1X49-D110.4 to 18.4R3-S2. You are expected to see an error "unsupported h/w" if you try the upgrade.
Even as per the KB link you mentioned in the post, you can upgrade the box from 15.1X49-D110.4 to 18.2R3 or 18.2R3 based Service Releases directly and from there you can upgrade to 18.4R3-S2 without any issues.
So please follow the step upgrade and you should not see any issues:
15.1X49-D110.4 ---> 18.2R3 or 18.2R3 based Service Releases ----> 18.4R3-S2.
Hope this helps.
Thanks and Regards,
Pradeep Kumar M
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I can see the below note in KB (https://kb.juniper.net/InfoCenter/index?page=content&id=kb21476😞
Notes for upgrading from Junos 15.1X49 releases to 18.2R3 or 18.2R3 based Service Releases:
Junos OS upgrade from 15.1X49 directly to 18.2R3 or 18.2R3 based Service Releases is supported for all SRX platforms, except vSRX. To upgrade vSRX from 15.1X49 to higher versions, deploy a new vSRX VM.
So, I guess there should be no any base OS requirements from 18.2R3 to 18.4R3-S2?
Thats true. You can directly upgrade from 18.2R3 to 18.4R3-S2. No issues.
As mentioned earlier, follow this: 15.1X49-D110.4 ---> 18.2R3 or 18.2R3 based Service Releases ----> 18.4R3-S2.