Reference Topology With Limited High Availability
A short list of important points
The limits
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In a Technet Article 
Reference Topology With Limited High Availability you can read that “If you want to accommodate more users or provide some high availability capability for Enterprise Voice (at a minimum cost), you could add another Standard Edition server to this topology.”

SORG Topology diagram

Lync Standard Edition has a lower cost than Lync Enterprise Edition, so this sounds really interesting.

The important point here is to understand how to make it work and which functions you’ll have up and running during a failover on a second S.E. server

The starting point is that you can configure a second S.E. servers as a backup registrar in case a failure happens on the primary registrar (it’s a “crossed” reference, so A has B as backup registrat and viceversa)

This new feature was introduced especially for voice resiliency. However, there are some features not available for the users connected to backup registrar. Please check the following thread for an overview.

(Here you have an interesting thread on Technet

 Also remember that

a) Lync topology is bounded by the forest. So, you’ll have to add the second S.E. server in the existing topology

b) You’ll have to configure a second S.E. pool so that your users can register on it if their primary pool is unavailable

c) You can create multiple pools, however only one pool in the forest will house the CMS.

d) Keep in mind that Administrators are not bound at the Lync pool level. Lync administrators will have the ability to enact changes that affect any pool, since the Lync infrastructure does not bound based on domains/pools. Additionally the two pools will attempt message passing between them, and users address lists will include users from the entire forest.

(Another interesting thread on Technet )

e) Non Voice and/or IM services are automatically collocated with the Standard Edition server.

 User services remains on his main pool and the failover and failback is only for the registrar which means client will be able to sign-in and do limited functionality (for a example : two party IM works with a backup registrar without contact list for the user)

The limits will stay until

a) the main pool goes up or

b) until after you move the users to the surviving pool

An example of the above configuration with two S.E. servers configurated as a couple of primary / backup registrars

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This article is available in other languages, including Italian.

Aggiungere un secondo server Lync Standard Edition per dare una (limitata) alta affidabilità (it-IT)