NimbostratusWe have two BIG-IP LTM 3600 running BIG-IP 11.2.1 Build 797.0 Final in Active/Standby cluster. What are the recommended firmware upgrades and implications associated? Thank you for your advice.
Cirrus11.5.x is the LTS (Long Term Support) version. I would recommend any version in this train. For a specific version, consider 11.5.4 with Hotfix2. HF2 was released 1-2 weeks ago.
MVPstarting 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.