# Limitations of Multi-AZ DB clusters for Amazon RDS
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A Multi-AZ DB cluster has a writer DB instance and two reader DB instances in three separate Availability Zones. Multi-AZ DB clusters provide high availability, increased capacity for read workloads, and lower latency when compared to Multi-AZ deployments. For more information about Multi-AZ DB clusters, see [Multi-AZ DB cluster deployments for Amazon RDS](multi-az-db-clusters-concepts.md).

The following limitations apply to Multi-AZ DB clusters.
+ Multi-AZ DB clusters don't support the following features:
  + IPv6 connections (dual-stack mode)
  + Cross-Region automated backups
  + Kerberos authentication
  + Modifying the port. As an alternative, you can restore a Multi-AZ DB cluster to a point in time and specify a different port.
  + Option groups
  + Point-in-time-recovery (PITR) for deleted clusters
  + Storage autoscaling by setting the maximum allocated storage. As an alternative, you can scale storage manually.
  + Stopping and starting the Multi-AZ DB cluster
  + Copying a snapshot of a Multi-AZ DB cluster
  + Encrypting an unencrypted Multi-AZ DB cluster
+ RDS for MySQL Multi-AZ DB clusters support only the following system stored procedures:
  + `mysql.rds_rotate_general_log`
  + `mysql.rds_rotate_slow_log`
  + `mysql.rds_show_configuration`
  + `mysql.rds_set_external_master_with_auto_position`
  + `mysql.rds_set_configuration`
+ RDS for PostgreSQL Multi-AZ DB clusters don't support the following extensions: `aws_s3` and `pg_transport`.
+ RDS for PostgreSQL Multi-AZ DB clusters don't support using a custom DNS server for outbound network access.