With the exception of the Archiving and Monitoring role and the standard edition server, all other Lync server roles can be deployed for high availability. The following sections describe the required additional components.

Standard Edition Server

The Standard Edition server combines many of the server roles on one server. High availability options are not available for the Standard Edition server therefore it is recommended to use Lync Server 2010 Enterprise Edition if a highly available solution is required.

Front-End Server and Back-End Server

To improve availability, Front-End servers are deployed in a pool. A Front-End pool is a set of Front-End servers configured identically, that work together to provide services for a common group of users. A pool provides scalability and failover capability to the users. If multiple servers are configured in a pool configuration, Hardware and or DNS load balancing is required to distribute the load and enable failover.

Increasing the availability of the Back-End servers can be achieved by deploying a cluster of two or more servers.

A/V Conferencing Server

Conferencing servers can either be deployed co-located on the Front End servers in a pool or as one or more dedicated servers. A co-located pool setup ensures high availability. In the case of a dedicated server deployment it is recommended to deploy at least two A/V Conferencing Servers for high availability.

Edge Server

For further details refer to: http://technet.microsoft.com/en-us/library/gg425716.aspx

If high availability is required, at least two Edge Servers should be deployed in a pool. A single Edge pool will support up to ten Edge Servers. If multiple servers are configured in a pool configuration, hardware and or DNS load balancing is required to distribute the load and enable failover.

Mediation Server

To improve availability, multiple Mediation servers can be deployed. The Enterprise Voice routing component will reroute voice traffic in the case of server or connection failure.

Monitoring & Archiving Server

A Monitoring and/or Archiving server outage will not negatively affect overall Lync service availability. Both server roles use Microsoft message queuing for data exchange and are therefore less susceptible to failure. If an Archiving or Monitoring server fails, messages will remain in the queue until the server is available again. The availability of the server role can be improved by upgrading the hardware specifications and clustering the Back-End database server. It is possible to configure the archiving service as critical for the Lync deployment; this will cause the Lync services to pause if the archiving service is unavailable for a longer period of time and prevent messages from not being archived. For this mode of operation a standby sever could be considered to improve availability.

Director

Similar to the Front End server, Director server availability can be increased by deploying multiple director servers in a pool. A pool of Directors must be load balanced by either a hardware load balancer or by implementing Domain Name System (DNS) load balancing to take care of the SIP traffic.