What is an availability group listener?

Article
03/03/2023

Applies to: SQL Server

An availability group listener is a virtual network name (VNN) that clients can connect to in order to access a database in a primary or secondary replica of an Always On availability group. A listener allows a client to connect to a replica without having to know the physical instance name of the SQL Server. Since the listener routes traffic, the client connection string doesn't need to be modified after a failover occurs.

An availability group listener consists of a Domain Name System (DNS) listener name, listener port designation, and one or more IP addresses. Only the TCP protocol is supported by availability group listener. The DNS name of the listener must be unique in the domain and in NetBIOS. When you create a listener, it becomes a resource in a cluster with an associated virtual network name (VNN), virtual IP (VIP), and availability group dependency. A client uses DNS to resolve the VNN into multiple IP addresses and then tries to connect to each address, until a connection request succeeds or until the connection requests time out.

If read-only routing is configured for one or more readable secondary replicas, read-intent client connections to the listener are automatically redirected to a readable secondary replica.

This article provides an overview of an availability group listener. You can also configure the listener, and then learn how to connect to the listener.

Listener parameters

An availability group listener uses the following:

A unique DNS name
This is also known as a Virtual Network Name (VNN). Active Directory naming rules for DNS host names apply. For more information, see the Naming conventions in Active Directory for computers, domains, sites, and OUs KB article.

One or more Virtual IP addresses (VIPs)
VIPs are configured for one or more subnets to which the availability group can fail over.

IP address configuration
For a given availability group listener, the IP address can use either Dynamic Host Configuration Protocol (DHCP), or one or more static IP address. Using DHCP can cause connectivity delays during failover, and so it is not recommended for use in production environments. Availability groups that extend across multiple subnets, or use hybrid network configurations, must use a static IP address.

Listener port

When configuring an availability group listener, you must designate a port via SSMS. You can configure the default port to 1433 in order to allow for simplicity of the client connection strings. This means, that if you use 1433, you don't need to include a port number in a connection string of your application. Also, since each availability group listener will have a separate virtual network name, each availability group listener configured on a single WSFC can be configured to reference the same default port of 1433.

If you use the default port of 1433 for availability group listener VNNs, you'll still need to ensure that no other services on the cluster node are using this port; otherwise this would cause a port conflict.

If one of the instances of SQL Server is already listening on TCP port 1433 via the instance listener and there are no other services (including additional instances of SQL Server) on the computer listening on port 1433, this won't cause a port conflict with the availability group listener. This is because the availability group listener can share the same TCP port inside the same process. However multiple instances of SQL Server (side-by-side) must not be configured to listen on the same port because one of them will fail to listen for connections.

You can also designate a non-standard availability group listener port. However, you also need to explicitly use the target port in your application connection string when connecting to a listener. You also need to open permission on the firewall for this port.

You can connect to the listener using the name and port (if not 1433). The port can be either the listener port or the underlying SQL Server port that it's configured to listen on.

The following examples demonstrate some of the functionality of the listener:

Set up:

IP that SQL Server is listening on: 192.168.20.2
Port that SQL Server is listening on: 50254
Listener IP that was configured: 192.168.20.15
Listener name was configured: aglistener19
Listener port that was configured: 50123

Connect to the listener via IP address and port. This connection is successful.
```
sqlcmd -S 192.168.20.15,50123 
1> 
```
Connect to the listener by name only, no port. This connection will fail because a non-default port was used. You must specify that port.
```
sqlcmd -S aglistener19 
```
Connect to the listener by listener name and configured port. This connection is successful.
```
sqlcmd -S aglistener19,50123 
1> 
```
Finally, connect to the listener and the SQL Server port. Notice that in this case you're using the port SQL Server is listening on, not the listener port. This connection also succeeds.
```
sqlcmd -S aglistener19,50254
1> 
```

Behavior of client connections on failover

When an availability group failover occurs, existing persistent connections to the availability group are terminated and the client must establish a new connection in order to continue working with the same primary database or read-only secondary database. While a failover is occurring on the server side, connectivity to the availability group may fail, forcing the client application to retry connecting until the primary is brought fully back online.

If the availability group comes back online during a client application's connection attempt but before the connect timeout period, the client driver may successfully connect during one of its internal retry attempts and no error will be surfaced to the application in this case.

Next steps

Now that you're familiar with how an availability group listener functions, create your listener and then configure your application to connect to the listener. You can also review various availability group monitoring strategies to ensure the health of your availability group.

For more information about availability groups, see the Overview of Always On Availability Groups (SQL Server).