SharePoint 2013 - External Content Types (BCS) - Part 2

This article will explain you how to use existing external content types.

Step 1 : Go to Central Admin and click on BDC service under Manage Service Application.
Step 2 : Select existing service and click on Set Object Permission

Step 3 : Add any existing user and set permission as below and click ok.

Step 4 : Go to any of existing web application.
Step 5 : Go to Site Content, click on Add an App and select External List.

Step 6 : Provide Name and External Content Type as per screenshot.

Step 7 : Do IISRESET and Navigate to that External List and you will see external data.

Make Sure MACHINENAME\IUSR has enough permision otherwise you will get "Message from External System : 'Login failed for user 'NT AUTHORITY\IUSR'.'." error.