BIG-IP LTM 3600 11.2.1 upgrade path

starting from 12.x there isn't one LTS version anymore, both 11.6.1 and 12.1 have longer supported times now then 11.5.4.

reference: http://support.f5.com/kb/en-us/solutions/public/5000/900/sol5903.html

still looking from a conservative point of view the smallest jump might cause the least problems. so if you don't have any need for 11.6 / 12.x feature it is a logical choice.

as for possible issues, read through the releases nodes and try to do your upgrade in a test environment first. for one things http classes will change into local traffic policies.