Cisco Nexus 9500 Parallel Upgrade Process

Cisco Nexus 9500 Series switches are modular EoR switches that require two supervisors for ISSU. The minimum configuration required is two system controllers and two fabric modules.

Cisco Nexus 9500 Series switches support parallel upgrade as the default method. The parallel method upgrades the modules in the batches (as outlined in the following illustration) instead of upgrading the modules one after the other.

Figure 1. Parallel Upgrade Process for Cisco Nexus 9500 Series Switches

The steps for the parallel upgrade process on Cisco Nexus 9500 Series switches are the following:

First, the supervisors are upgraded (This procedure requires a switchover). Then the line cards, the fabric modules, the system controllers, and the FEX are upgraded.
After the switchover is performed in a parallel upgrade, the secondary supervisor takes over. The installer determines the current line cards and the fabric modules.
The installer then divides the components into buckets. It places the first half of the line cards in the first bucket, the first half of the fabric modules in the second bucket, the second half of line cards in the third bucket, the second half of the fabric modules in the fourth bucket, the first system controller in the fifth bucket, and the second system controller in the sixth bucket.
Each bucket is upgraded successfully before an upgrade process starts for the next bucket.
The console displays the modules with the bucket assignments and the status of the upgrade.

While performing standard ISSU for the modular switches, the data plane traffic is not disrupted. The control plane downtime is approximately less than 6 Seconds.

Reference:

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus9000/sw/7-x/upgrade/guide/b_Cisco_Nexus_9000_Series_NX-OS_Software_Upgrade_and_Downgrade_Guide_Release_7x/b_Cisco_Nexus_9000_Series_NX-OS_Software_Upgrade_and_Downgrade_Guide_Release_7x_chapter_010.html