## In-Service Software Upgrade in a Two-Node Cluster

In-Service Software Upgrade (ISSU) is a comprehensive, transparent software upgrade application that allows you to deploy bug fixes and add new features and services without any disruption to the traffic.

In a cluster consisting of the MDS 9222i switches as members, if the switches are not able to communicate, then the switch having the lowest node identifier (node ID) remains in the cluster while the other switch leaves the cluster. However, when an ISSU is performed on a switch having the lowest node identifier, a complete loss of the cluster results because both the switches leave the cluster.

This undesirable situation is addressed in a two-switch cluster as follows:

• The upgrading switch sends a message to the other switch of the intent to leave the cluster. The upgrading switch can either be a master switch or a slave switch.

• The remaining switch remains in the cluster and performs the role of the master switch if it was a slave switch. This switch continues to remain in the cluster with the quorum intact.

• After the ISSU is completed and the switches boot up, the upgraded switch rejoins the cluster as a slave switch.

Note This feature is tied to the internals of ISSU logic and no additional commands need to be executed.