After an ONTAP upgrade, changes in security protocols or configuration syntax can affect external service integrations like LDAP. If a user cannot log in, the issue is often with the fundamental connection and search parameters used to communicate with the LDAP server. The LDAP bind distinguished name (DN) and base DN are critical for authenticating the storage system to the LDAP server and specifying where to begin searching for user accounts. An incorrect or outdated configuration in these fields is a common point of failure.

A. DNS cache size: While DNS is critical for resolving the LDAP server name, its cache size is unlikely to be the direct cause of login failures post-upgrade.

C. Aggregate encryption key: This is related to NetApp Aggregate Encryption (NAE) for data-at-rest and is not involved in the LDAP authentication process.

D. Cluster peer certificate: This is used for securing communication between peered clusters, not for authenticating users against an external LDAP server.

1. NetApp ONTAP Documentation. (2023). ONTAP 9 Name Services Management Guide. Section: "Troubleshooting LDAP". (Outlines checking the LDAP client configuration, including bind-dn and base-dn, as a primary step).

2. NetApp Knowledge Base. (2021). Article 1074513: How to troubleshoot LDAP authentication failures in ONTAP. This article lists verifying the vserver ldap client show output, which contains the base and bind DNs, as a key diagnostic step.