The customer has MX104 router runing on Junos 15.1R7 (which is recommend version as of now). This Junos version will go EOS on 05-Dec-2019 (see EOL dates). I was asked to check for them the recommended upgrade path to other JTAC recommended release 17.3R3.
The wording in upgrade support policy (here) in Junos release notes for 17.3R3 is bit confusing for me as non native speaker. I understood that I can upgrade up to two EEOL release apart.
Junos EOL doc gives me impression that the EEOL releases for MX104 are 15.1R7 -> 16.1R7 -> 17.3R3 (16.2 is not EEOL for MX104, only for certain platforms). Customer should be able to upgrade in one step - from 15.1R7 directly to 17.3R3 based on this (two EEOL releases apart).
Can somebody confirm it / comment further please? I'm not aware if they have single or dual RE setup or features used.
I asked to our Juniper SE but he seems to be overloaded at the moment.
I agree with your assessment - your customer should be able to upgrade from 15.1R7 to 17.3R3 in one step without any issues. In practice, it's very possible that there won't be any issues even if he upgrades to something newer than 17.3R3, but Juniper upgrade policy is there to try to make sure that everything will still be working.
It's always a good idea to check it in the lab first (and to prepare and validate corresponding MOP). It may save a lot of time later if something goes wrong during the upgrade, or if urgent downgrade is needed.
I agree with you on all points. Customer should be able to upgrade safely even to higher Junos release (especially if they are using base functionalities only). I just wanted to be as safe as possible for the customer. Lab upgrade and detailed MOP are always a good idea but this responsibility will be with customer.
There is no change to underlying FreeBSD version between those two Junos release on MX104. This should make the upgrade less problematic too.