Network Load Balancing comes in a few different ways:
- Hardware based Network Load Balancers (typically an appliance, but could be a module for switch/router)
- Software based Network Load Balancers (typically deployed as a VM template on a hypervisor such as Hyper-V)
- Windows Network Load Balancer (a component of Windows Server), Not supported for use with Lync Server
- DNS Load Balancing (specific to Lync servers and clients)
If you have a Lync environment any larger than a few servers then you will probably require a load balancer, which can provide high availability for the following server roles/pool:
- Front-End Server Pool
- Director Server Pool
- Mediation Server Pool
- Edge Server Pool
- Office Web App Servers
DNS Load Balancing or not, that be the question
DNS Load Balancing (DNS LB) from a DNS admin perspective just looks like a round robin address however is application assisted and is principally is only supported with Lync servers and Lync clients as well as Exchange Server after 2010 SP1 and also qualified gateways and SBA’s.
DNS load balancing has the advantage of server drain which a Lync admin can easily initiate otherwise with NLB, a Lync admin needs to speak with the NLB admin to place a node into maintenance/drain model when preforming maintenance on a node within the server pool.
Regardless of whether you use DNS Load Balancing or not you will need to use a full network load balancing (NLB) for web services for Front-end and Director Server Pools.
However before you make a decision, let look at some of the key decision points and whether you think high availability is important.
Front-End and Director Server Pools
- Clients: if will be supporting older and 3rd party client
- Migration: you are migration from OCS and will be in a long term coexistence, therefore larger number of user will be homed on OCS still
- Remote Call Control (RCC): it is very largely the CSTA gateway won’t support DNS NL, therefore for high availability you will need full NLB for the signalling.
• Running UCMA applications prior to version 3.0
Mediation Server Pool
- Integrating with Non-Qualified Gateway (not supporting DNS load balancing) although you could potentially integrate with the mediation server separately
Edge Server Pool
- Federation with earlier version (LCS/OCS)
- Public IM Connectivity with AOL, Yahoo and Windows Live
- You are running an Exchange server prior to 2010 SP1, other remote user will potentially have issue with Playing their Enterprise Voice voicemail on their phone and Transferring calls from an Exchange UM Auto-Attendant
- XMPP, with Lync Server 2013
You can’t have your cookie and eat it!
With the introduction of CU4 and the mobility service with Lync Server 2010, external web services is now required to use cookie presence for load balancing and if your connecting devices running Lync Mobile to your internal wireless network and they will be connecting to your front-end pool directly then cookie presence must be used for internal web services 
With Lync Server 2013 the usage of cookie presence is greatly reduced (more about this in a upcoming question)
 Hardware Load Balancer Requirements for Lync Server 2010