The FEX does not reload , it will be upgraded without a reload. Please look at the following link:
http://www.cisco.com/en/US/docs/switches/datacenter/nexus5500/sw/upgrade/602_N1_1/n5500_upgrade_downgrade_602.html#wp687951
Impact of Upgrades and Downgrades
Cisco NX-OS supports in-service software upgrades (ISSUs) that allow a Cisco Nexus device and any connected FEXs to be upgraded without any traffic disruption (with a brief control plane disruption). A few conditions have to be met for the system to be upgraded via ISSU process - the access layer topology should be ISSU compliant, the current and target versions should be ISSU capable and the network should be stable.
If the conditions required for ISSU are not met or if the user intends to downgrade the software version, the installation process is disruptive. For example the Cisco Nexus device and any connected FEX are rebooted, which causes a disruption. If Cisco's virtual port channel (vPC) is configured on Cisco Nexus devices, it is possible to achieve an upgrade/downgrade with very minimal traffic disruption to servers/hosts.
In-Service Software Upgrades
With a single supervisor system like a Cisco Nexus device, an ISSU on the Cisco Nexus device causes the supervisor CPU to reset and load the new software version. The control plane is inactive, but the data plane keeps forwarding packets that lead to an upgrade with no service disruption. After the CPU loads the updated version of Cisco NX-OS, the system restores the control plane to a previously known configuration and the runtime state and the data plane are synchronized. Because the data plane keeps forwarding packets while the control plane is upgraded, any servers connected to the Cisco Nexus device access layer should see no traffic disruption.
ISSU Support For Cisco Nexus Fabric Extenders
Cisco Nexus Fabric Extenders act as line cards to Cisco Nexus devices. The fabric extenders add flexibility to datacenter networking infrastructure by decoupling the physical and logical (Layer 2) topology, reducing the operation expense by lowering management and troubleshooting points, and building a larger layer 2 fabric that is loop-free, with a single layer of switching.
The ISSU process initiated on the Cisco Nexus devices upgrades the entire access layer including the switch and the FEXs that are connected to the switch.
An ISSU first upgrades the switches. Once the switch is operational with the upgraded software, the FEX's are upgraded. This upgrade of the FEX is done in a rolling fashion, one FEX at a time. This upgrade on the Cisco Nexus Fabric Extenders is non-disruptive, similar to the upgrade of the switch.
The time required for an ISSU to complete depends on the number of FEX's that are connected. You should plan a maintenance window with the total upgrade time in mind. The entire upgrade is non-disruptive and is not expected to cause any outage to connected servers.
Hope this helps.
JayaKrishna
