I want to federate my ArcGIS Server site with Portal for ArcGIS. Do both of these products need to be running the same version?

Yes. To federate your ArcGIS Server site with Portal for ArcGIS, both products need to be at the same release version.

Can I configure an earlier version of ArcGIS Web Adaptor with Portal for ArcGIS?

No. To configure ArcGIS Web Adaptor with Portal for ArcGIS, both products need to be running the same release version.

Can I install ArcGIS 10.3.1 Web Adaptor (IIS) on the same machine as an earlier version of ArcGIS Web Adaptor (IIS)?

Yes. Beginning at 10.2.1, you can have a 10.2.1 or later version of ArcGIS Web Adaptor (IIS) installed alongside one other earlier version of Web Adaptor on the same machine. For example, you can have a 10.1 SP1 version and a 10.3.1 version of Web Adaptor installed on the same web server.

As with earlier releases, the Web Adaptor version must always match the version of Portal for ArcGIS. For example, you cannot configure a 10.3.1 version of Web Adaptor with an earlier version of Portal for ArcGIS.

To upgrade an earlier version of ArcGIS Web Adaptor (IIS) to 10.3.1, you need to uninstall the earlier version and install the 10.3.1 setup. After installing 10.3.1, you need to configure Web Adaptor with Portal for ArcGIS.

To learn more, see Installing ArcGIS Web Adaptor (IIS).

Can I use ArcGIS 10.2, 10.2.1, or 10.2.2 for Desktop to publish or administer items running on Portal for ArcGIS 10.3.1?

Yes. You can use the 10.2, 10.2.1, or 10.2.2 version of ArcGIS for Desktop to publish or administer items running on Portal for ArcGIS 10.3.1.

Conversely, you cannot use a 10.1 SP1 or earlier version of ArcGIS for Desktop to publish or administer items running on Portal for ArcGIS 10.3.1.

Using an earlier version of ArcGIS for Desktop, can I view and use services running on Portal for ArcGIS 10.3.1?

Yes. In most cases, you can add 10.3.1 services to earlier versions of 10.x ArcGIS for Desktop. The fundamental structure and format of web service requests understood by services have not changed at version 10.3.1. However, if you're connecting from a pre-10.2 version of ArcGIS for Desktop, you must connect to Portal for ArcGIS, not the federated ArcGIS 10.3.1 for Server site, to view the services.

Be aware that you can only connect from earlier versions of ArcGIS for Desktop to Portal for ArcGIS 10.3.1 using connection options that are supported in the ArcGIS for Desktop version you're using. For example, you cannot use integrated Windows-authenticated logins to connect to Portal for ArcGIS 10.3.1 from ArcGIS 10.1 for Desktop, because integrated Windows-authenticated logins were not supported in ArcGIS 10.1 for Desktop.

What is the migration path from earlier versions of Portal for ArcGIS to version 10.3.1?

Running the 10.3.1 setup will automatically upgrade your existing deployment of Portal for ArcGIS to 10.3.1. The upgrade does not migrate any custom behavior you've configured for your portal, such as properties you've modified in config.js. You are responsible for backing up and restoring any customization between versions. To learn how to upgrade an earlier version of Portal for ArcGIS to 10.3.1, see About upgrading Portal for ArcGIS.

Migration to Portal for ArcGIS 10.3.1 from a pre-10.2 version is only supported through an Esri Professional Services engagement.