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Now that the Front End Servers are functioning correctly, you can install and configure your load balancer. First, confirm that your load balancer meets the requirements for Office Communications Server load balancers, which are described in Load Balancers for Office Communications Server 2007 R2in the Technical Reference for Office Communications Server 2007 R2. Next, identify the load balancer network topology used in your environment:

• One-armed: In this topology, the Office Communications Server Front End Servers and clients reside on a single network. The load balancer has a single arm, connected to this same network.
  
  
• Two-armed (routed): In this topology, the Office Communications Server Front End Servers and clients reside on two routable networks. The load balancer has two arms, one connected to the client network and the other connected to the network with the Office Communications Server Front End Servers.
  
  
• Out-of-path mode: This is a form of the one-armed topology in which the initial client request passes through the load balancer, but return traffic travels directly from the Front End Server to the client. This mode is not supported by Office Communications Server.
  
  

The mechanism to configure the load balancer varies by manufacturer, but the following core steps are required no matter which model is used:

Connect Load Balancer and Confirm Networking

If your environment uses a one-armed topology, connect the load balancer to the corporate network where your clients and Office Communications Server Front End Servers are located and configure a single virtual IP address (VIP). This static IP address is used by Office Communications Server clients and Front End Servers to provide a single entry point for connecting to resources provided by the Front End Server array. In our sample one-armed topology, the VIP is 10.0.0.40.

If your environment uses a two-armed topology, connect one arm of the load balancer to the corporate network and the other arm to the network where your Office Communications Server Front End Servers reside. Next, configure a VIP on the client network and a corresponding IP address on the Front End network. In our sample two-armed topology, 10.0.0.40is the pool VIP and 192.168.0.40is the corresponding IP address on the Front End network.

Configure load balancer with Front End Pool Servers

Now that the virtual IP has been created, the next step is to configure it to point to the newly created Front End Servers. In our sample one-armed topology, the 10.0.0.40VIP would be configured to use 10.0.1.41and 10.0.1.42.

Besides pointing to the IP addresses of the Front End Servers, a number of additional settings need to be enabled in the VIP configuration to ensure proper operation with Office Communications Server:

1. Ensure that the VIP supports the following TCP ports: 5060, 5061, 135, 443, 444, 5065, 5069, 5071, 5072, 5073, 5074, and 8404. For details about each port, see Load Balancers for Office Communications Server 2007 R2in the Technical Reference for Office Communications Server 2007 R2.
   
   
2. Configure the VIP to use a TCP idle timeout of 30 minutes.
   
   
3. Configure the VIP to use a weighted least connectionsalgorithm in choosing how to load balance incoming connection requests against the Front End Server array.
   
   
4. Configure the VIP with a heart beat monitor, which polls each Front End Server on ports 5060 (SIP over TCP), 5061 (SIP over TLS), and 444 (conference state over HTTP). This enables the load balancer to detect when one of the Front End Servers goes down and take that server out of the array.
   
   
5. Configure the VIP to use Source Network Address Translation (SNAT) mode. This means the load balancer uses one of its IP addresses as the source IP address when it sends the connection to one of the Front End Servers. In Destination Network Address Translation (DNAT) mode, the load balancer uses the source IP address of the endpoint that originated the connection when it sends a connection to one of the Front End Servers. DNAT mode is not supported.
   
   

   Only SNAT is supported in Office Communications Server 2007 R2 because of the relative complexity required to set up DNAT and to configure it properly and because of the way Office Communications Server R2 Front End Servers behave behind a load balancer configured with to use DNAT. Unpredicable behavior can result when Front End Servers communicate and simultaneously try to load balance traffic within an Enterprise pool.

6. If your pool will require more than 65,000 simultaneous connections, configure additional SNAT IP addresses on your VIP. This is because the load balancer would only have one IP address configured on the network containing the Office Communications Server Front End Servers. This would limit the load balancer to roughly 65,000 source ports when making connections to the Front End Server.
   
   

It is very important that all of the requirements and configurations listed above be completed correctly. When you are finished, you will validate the load balancer configuration.