The RAP needs to go through a upgrade hop in order to come up on 6.x controller "3.x -> 5.x -> 6.x"
Following are the sequence of steps:
1. Terminate the RAP-5WN on a controller running 5.x code version
2. Upgrade the backup-partition on the RAP to the 5.x code version using the following command on the 5.x controller
apflash ap-name <Name_of_RAP> backup-partition
3. Run the following command to make sure that the backup partition on the RAP has been upgraded to 5.x version
show ap image version ap-name <ap-name>
4. Now, re-provision the RAP to connect to 6.x controller.
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HT