How can ISSU be nondisruptive for MDS 9148 or 9396?

[tenaro.gusatu.novici]

How can In-Service Software Upgrade be nondisruptive if switches have no redundancy for control plane or data plane? Documentation states that MDS 9148 (or MDS 9396) support nondisruptive ISSU process and it also states that fabric switches have a single control plane and a non-redundant data plane. 

I can understand that mission-critical directors are capable of ISSU because they can have two supervisors and multiple crossbar fabric modules.

[Kirk J]

Greetings.

From https://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/6_2/upgrade/guides/nx-os/upgrade.html#22915

Preparing for a Nondisruptive Upgrade on Fabric and Modular Switches

You can upgrade software on the following switches without any disruptions by using the install all command for the system software images:

• Cisco MDS 9148 Multilayer Fabric Switch
• Cisco MDS 9148S Multilayer Fabric Switch
• Cisco MDS 9222i Multiservice Modular Switch
• Cisco MDS 9250i Multiservice Modular Switch

When the installation is completed, the supervisor kickstart image, supervisor system image, line card image, and the system BIOS are all updated.

Nondisruptive upgrades on these fabric switches disrupts the control plane for not more than 80 seconds. The software upgrade might be disruptive if the upgrade progresses beyond when it can be stopped gracefully or if a failure occurs.

Note During the upgrade the control plane is down, but the data plane remains up. New devices are not able to log in to the fabric through the control plane, but existing devices do not experience any disruption of traffic through the data plane.

Thanks,

Kirk...