Oracle Database Hang, no one was able to connect

What is the problem?

I received an email from the client that no one was able to connect to the database this morning (after 8:20 am).
This case is a good example to show you how it is important to collect information about the issue from the concerned people before diving into the log files and trace file.

Environment

This is an Oracle RAC cluster, 2-nodes, Oracle 19c, IBM-AIX

Analysis

Let us see what is there in Node #1, using the adrci utility:

Let us see the details of the last incident:

What is there in the alert.log file of instance#1?

Let us review Node #2 also:

What is the Oracle Hang Management?:

According to Oracle:

What is the Root Cause of this issue?:

Based on the above analysis, we saw that there are couple of hangs, but we didn’t reach the root cause of these hangs.
But when I talked the customer DBA , he told me that they restarted the main network switch !!!!!!!!!!!!!!!!!
So, this was the root cause of the issue !!.