Replace F5 8900 version 11.6.1 by F5 iR800 version 12.1.2

Good morning,

Be certain to disable the interfaces on the device(s) that will be running 12.1.2. If you don't, you may be introducing a condition where there are two F5s ARPing VIP addresses into the environment.

Check the following solution for a similar question posted a few days ago:

https://devcentral.f5.com/questions/migrating-from-bigip-2000-series-to-bigip-2600-series-60044