Hello M S K,
The behavior when connecting the fex uplinks from 5020 to the 5596, will be all the discovery phase to bring the FEX online on the new n5k, this will cause the fex ports to go UP and down and back up, as well as that both parent n5ks should have the same firmware and nx-os versions, otherwise the FEX will see that it needs to upgrade or downgrade its firmware.
So long story short, to answer your question:
what kind of precautions should be taken care?
You would probably do this during a maintenance window, first off power off the fex switches and migrate the uplinks to the new n5ks dual homed, then power up the fex switches and when they come up, they will go through the doscovery phase, and probably the upgrade of the firmware that take between 10 to 15 mins. Then you will see in the logs "Module Online" and with "show fex ### detail" you can see the state of the Fex switch. This is a simpler process so that you don have to downgrade the version of the software.
Make sure that the config for the fex is aloready provisioned in the new n5ks, so that you assure to yourself a plug and play as to configuration:
https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/system_management/502_n1_1/b_Cisco_n5k_system_mgmt_cg_rel_502_n1_1/Cisco_n5k_system_mgmt_cg_rel_502_n1_1_chapter4.html
Keep us posted, if this helped you please rate all helpful answers and mark as correct,
Regards,
David castro,
