![]() This parameter determines whether the Windows Cluster will register all of the IP addresses the AGL is dependent on, or only the one active IP address. The first parameter of interest is called By modifying these parameters on the server we can transparently change the experience of the client OS in its name resolution caching. There are two parameters that affect how the AGL is registered with DNS. If you are unsure, please check with your DNS administrators to determine if dynamic updating of DNS is allowed. ![]() This assumes that dynamic updating of DNS is allowed within the environment. One option to resolve the issue if an application cannot use the MultiSubnetFailover parameter is to change the behavior of how the AGL is registered with DNS. In some cases, the connection timeouts can be intermittent or they can be very consistent - depending upon the order in which IP addresses are returned. Sometimes it isn’t possible to modify the connection strings – and so some applications will still encounter timeout issues when trying to connect. The connection strings must be modified in order to ensure successful consistent and successful connections to a multi-subnet listener. The client libraries by default do not enable this parameter (i.e. The server will respond immediately and establish a connection, while the other IP addresses and their respective connection attempts will eventually timeout – but since the application is already connected it does not matter that those connection attempts timeout and fail. This means that whichever IP address is online will be attempted immediately rather than waiting for any timeouts on IP addresses that are not online. That is, all of the IP addresses that the availability group listener is dependent on will receive a SYN request at the TCP layer “in parallel” (technically one immediately after the other, but not waiting for acknowledgement – so effectively “in parallel”). It will no longer attempt all of the IP addresses serially, but in parallel. , should be used and set to “TRUE.” When set to TRUE, the connection attempt behavior changes. NET libraries with hotfixes – see below in Appendix A), Microsoft added a new connection string parameter that can be added to change the connection behavior. , therefore, some applications may experience intermittent connection timeouts – or delays in connecting – which can cause application delays or performance issues.īeginning with the native client libraries for SQL 2012 as well as the. The default connection timeout period for. For each subsequent IP address, it will again have to wait 21 seconds before moving to the next IP address until the connection attempt times out or it establishes a connection to an IP address that responds. The default timeout for a TCP connection attempt is 21 seconds and if the first IP address attempted is not online, it will wait 21 seconds before attempting the next IP address. This can be problematic, because depending upon DNS configurations, the “correct” or “online” IP address may not be the first IP address returned. Because not all of the IP addresses returned by DNS will be online, client applications can run into problems when attempting to connect to the listener.īy default, the behavior of the SQL client libraries is to try all IP addresses returned by the DNS lookup - one after another (serially) until the all of the IP addresses have been exhausted and either a connection is made, or a connection timeout threshold has been reached. The other listener IP address(es) will be offline. The listener IP address in the subnet currently hosting the availability group primary replica will be online. When a client operating system (OS) needs to resolve the AGL name to IP by querying the DNS server, the DNS server will return multiple IP addresses – one for each subnet. (If non Microsoft Windows DNS servers are used, the exact implementation can be different). The cluster will submit all of the IP addresses that are in the dependency list and the DNS server will generally register an A record for each IP address. By default, when it is brought online it will be registered in DNS by Windows Cluster. When the availability group listener (AGL) is configured properly, it will have an IP address for each defined subnet and have an “OR” dependency on each of the IP addresses. Usually there are only 2 subnets, however, there can be more. One of the issues that generates a lot of call volume we see on the AlwaysOn team is dealing with connectivity issues to the availability group listener in multi-subnet environments.Ī “multi-subnet” environment is defined when the OS cluster used as the backbone for AlwaysOn has server nodes that are located in multiple, different subnets.
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