Tuesday, October 8, 2013

DNS load balancing for Lync Server

In Microsoft Office Communications Server (OCS) 2007 R2 and earlier, the only supported load balancing solution is a hardware load balancer. DNS load balancing is introduced in Microsoft Lync Server 2010. The mail function of DNS load balancing is to provide a native load balancing mechanism option in Lync Server. DNS load balancing in Lync Server 2013 hasn’t changed much from that in Lync Server 2010.

What is Lync DNS load Balancing:
Lync DNS load balancing balances the network traffic that is unique to Lync Server, such as SIP traffic and media traffic. DNS load balancing is implemented at the application level in both server and client. The front end server register their fully qualified domain name as A record in DNS, when enterprise pool is created, the pool FQDN is registered to return from DNS the list of IP addresses of all the front-end servers. The client attempts to connect to one of the IP addresses that were returned. If the connection fails, the client attempts to connect to the next IP address in the list until the connection succeeds.
However, all HTTP and HTTPS traffic still need to be balanced by a supported hardware or software based solution. Lync DNS load balancing is supported for Front End pools, Edge pools, Director pools and stand -alone Mediation pools.
How Does Lync DNS Load Balancing Work
Lync DNS load balancing is not the traditional round robin DNS. Here the best example for show the difference between. Suppose that we have two Lync Server 201 Front End Server, each having their own respective IP address. The first Front End Server has an IP address of and second Front End Server has an IP address of
In round robin DNS, a Lync client would get back only one IP address when it is requesting to sign-in requests. The first client would receive the IP address, the second client would get, and the third client would receive

Lync Server’s way of handling DNS load balancing follows a different pattern. First we need to configure a pool for DNS load balancing. Each Front End server in the pool must have FQDN and the pool must also have a FQDN (Pool.Tech.com), which resolves to the physical IP addresses of the servers in the Pool. Finally, the pool’s Web Services needs a separate FQDN (Webinternal.Tech.com), which resolves to the virtual IP (VIP) address of the pool.
IP Addresses

Each Lync client get back the IP addresses of all Front End Server when it is requesting to sign-in, so each client get back the IP addresses of and and client would then try one of the IP addresses if connection fails, it would try the other IP address.
DNS load balancing is only part of the client connectivity matrix. The internal hashing and distribution of the client registration information is the other part, the two mechanisms work together to determine how a client connects to a pool.
DNS load balancing is not support for load balancing Web traffic. As a result, hardware load balancing is still required for load balancing Web traffic (such as address book services) in Lync Server 2010.

Limitations of Lync DNS Load Balancing
Every technology having some limitation, so Lync DNS load balancing having also some below limitation
Legacy clients and servers work on limited degree in a pool set up for DNS load balancing. Clients are able to connect to the first server that responds to the DNS query. However, if the first server that responds is not available, the client or server won’t be able to route the additional IP addresses that are available.
If Lync Server integrated with Exchange server for the purpose of providing unified messaging capabilities, need to run Exchange Server 2010 SP1 or later to leverage Lync DNS load balancing.
The internal and external interfaces of an Edge pool must use the same type of load balancing. It can’t be mismatched.
Using DNS load balancing on your Edge Servers causes a loss of failover ability in the following scenarios:
•Federation with organizations that are running versions of Office Communications Server released prior to Lync Server 2010.
•Instant message exchange with users of public instant messaging (IM) services, such as Windows Live, AOL, and Yahoo!, in addition to XMPP-based providers and servers, such as Google Talk.

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