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Cisco Nexus switches: "ISSU" vs "disruptive upgrade"

July 27, 2023 ,

Cisco Nexus switches: "ISSU" and "disruptive upgrade"

There are two different methods of performing a software upgrade on the Cisco Nexus switch. These are ISSU (In-Service Software Upgrade) and Disruptive Upgrade. 

Cisco Nexus switches: "ISSU" and "disruptive upgrade"
Fig 1.1- Cisco Nexus 9500

Let's explore each of these methods ISSU vs Disruptive upgrade:

ISSU (In-Service Software Upgrade):

ISSU is a feature that enables a Cisco Nexus switch to be updated to a new software version without affecting the data or control planes. This implies that the switch keeps forwarding traffic and keeping its current routing and switching state during an ISSU. ISSU does this by carefully organizing the upgrading process and ensuring that the new software version smoothly replaces the old one.

Benefits of using ISSU upgrade

  • During the upgrading process, network services are disrupted as little as possible.
  • There will be no effect on existing data plane and control plane activities.
  • During the update, high availability is maintained, making it suited for mission-critical situations.
  • Throughout the upgrading process, ISSU ensures high availability. ISSU contributes to the preservation of redundant pathways, protocol adjacencies, and other HA methods by guaranteeing that the control plane and data plane are not disrupted. This reduces the risk of downtime and potential service interruptions by preventing failover occurrences and keeping the network functioning smoothly.
  • Because ISSU avoids a full switch reload, it often results in a speedier upgrading procedure when compared to disruptive upgrades. This saves time and shortens the upgrade's maintenance timeframe.
  • ISSU streamlines the update process by eliminating the need for network managers to manually coordinate with various devices or undertake complex failover procedures. The ISSU procedure is usually automated and transparent, reducing the possibility of human mistake during the update.
  • The ability to upgrade using ISSU ensures that the network may take advantage of the most recent software improvements, bug fixes, and security patches while maintaining a high degree of dependability and availability.

Disruptive Cisco Nexus upgrade

As the name implies, a disruptive upgrade is a software upgrading approach that creates a momentary disturbance in the operation of the switch during the upgrade process. A disruptive upgrade involves taking the switch offline, replacing the old software version with the new one, and then bringing the switch back online with the new software running.

Benefits of using Disruptive Cisco Nexus upgrade

  • In some circumstances, disruptive upgrades are simpler and easier to conduct than ISSU.
  • Disruptive updates are now accessible on a wider variety of switches and software versions.

Which one to use ?

The decision between ISSU and disruptive upgrading is determined by the network environment's unique requirements and limits. ISSU is preferred in cases where continuous network availability is crucial. A disruptive upgrade, on the other hand, is a possible alternative in less critical contexts or where ISSU is not supported on the platform.

Disruptive upgrades can cause network re-convergence, requiring protocols like as OSPF, BGP, and Spanning Tree Protocol (STP) to recalculate pathways and re-establish adjacencies. ISSU eliminates this re-convergence procedure, which might take time and cause brief network instability.

To guarantee a successful and easy update process, it is critical to check the release notes, compatibility matrix, and upgrade guidelines supplied by Cisco for the specific Nexus platform and software version.

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