Info from email:

In this document:
https://access.redhat.com/documentation/en-us/red_hat_amq/7.3/html-single/using_amq_interconnect/index
in section:
3.5.2 -> securing incoming connections
In subsection: Enabling user name and password authentication:

It tells you that this is the default,

The SASL configuration file is configured.

By default, this file should be /etc/sasl2/qdrouterd.conf.

But in the next step it tells you:

In the router section, specify the path to the SASL configuration file.

router {
mode: interior
id: Router.A
saslConfigDir: /etc/sasl2/
}
saslConfigDir
The absolute path to the SASL configuration file that contains the path to the SASL database that stores the user names and passwords.

This made me think that the only router that would be reading the /etc/sasl2/qdrouterd.conf file I had just created was going to be the router where I set the "saslConfigDir: /etc/sasl2/" attribute. But what happened was that all my routers started reading it even without specifying the "saslConfigDir:" attribute because it is the default.