Choosing a Topology

Topic Last Modified: 2011-03-23

When you choose a topology, you can use one the following supported topology options:

Single consolidated Edge using network address translation (NAT)
Scaled consolidated Edge using NAT and Domain Name System (DNS) load balancing
Scaled consolidated Edge using public IP and hardware load balancing

The following table summarizes the functionality available with the three supported Microsoft Lync Server 2010 communications software topologies. The column headings indicate the functionality available for a given Edge configuration option. Using the Scaled Edge (DNS load balanced) option as an example, you can see that it supports high availability, requires network address translation of Edge external interfaces, , reduces cost because a hardware load balancer is not required and does not support failover for Exchange Unified Messaging (UM), public instant messaging (IM) connectivity, and federation with servers running Office Communications Server.

Summary of Edge Server Topology Options

Topology	High availability	NAT required	Additional external DNS A record required for each Edge Server in the Edge pool	Failover*
Single Edge	No	Yes	No	No
Scaled Edge (DNS load balanced)	Yes	Yes	Yes	No
Scaled Edge (hardware load balanced)	Yes	No	No (only one per VIP)	Yes

* Failover for Exchange UM (remote user), public instant messaging (IM) connectivity, and federation with servers running Office Communications Server.

Note:
The NAT required and Additional external DNS A record required for each Edge Server in the Edge pool columns pertain only to the Edge external interfaces. A Yes in the NAT required column means that the associated Edge Server topology supports the use of NAT on the Edge external interfaces. If you decide to use NAT, you must use it on all three external interfaces. A Yes in the Additional external DNS A record required for each Edge Server in the Edge pool column means that you need a DNS A record (and associated public IP address) in your external DNS for each Edge Server added to your Edge pool.

Note:

The NAT required and Additional external DNS A record required for each Edge Server in the Edge pool columns pertain only to the Edge external interfaces. A Yes in the NAT required column means that the associated Edge Server topology supports the use of NAT on the Edge external interfaces. If you decide to use NAT, you must use it on all three external interfaces. A Yes in the Additional external DNS A record required for each Edge Server in the Edge pool column means that you need a DNS A record (and associated public IP address) in your external DNS for each Edge Server added to your Edge pool.

The primary decision points for topology selection are high availability and load balancing. The requirement for high availability can influence the load balancing decision.

High availability If you need high availability, deploy at least two Edge Servers in a pool. A single Edge pool will support up to ten Edge Servers. If more capacity is required, you can deploy multiple Edge pools. As a general rule, 10% of a given user base will need external access.
Hardware load balancing Hardware load balancing is supported for load balancing Lync Server 2010 Edge Servers when using publicly routable IP addresses for the Edge external interfaces. For example, you would use this approach in situations where failover is required for any of the following applications:
- Public IM connectivity
- External access to Exchange 2007 Unified Messaging (UM) or Exchange 2010 UM
- Federation with companies running Microsoft Office Communications Server 2007 or Microsoft Office Communications Server 2007 R2
These three applications will continue to operate, but they are not DNS load balancing aware and will only connect to the first Edge Server in the pool. If that server is unavailable, the connection will fail. For example, if multiple Edge Servers are deployed in a pool to handle the federated traffic load, only one access proxy actually receives traffic while the others are idle.

DNS load balancing DNS load balancing is the recommended approach for load balancing Lync Server 2010 Edge Servers when using NAT for the Edge external interfaces.

Important:
Using DNS load balancing is recommended if you are federating with companies using Lync Server 2010. Be aware that there are significant performance impacts if most of your federated partners are using Office Communications Server 2007 or Office Communications Server 2007 R2.