In Lync 2010 the site resiliency was given by stretching one FE pool across 2 sites, however this setup was much complicated during disasters and hence it is not supported and was discontinued from Lync 2013.
In Lync 2013 there is a new concept called pool pairing by which we can have datacenter resiliency by creating a second Enterprise pool and fail-over to that pool in an event of primary datacenter failure. So basically you will need to create two enterprise FE pool in your topology, one pool in primary site & second pool in DR site.
Below are the steps to configure pool pairing in Lync 2013
1. In Topology Builder, right-click one the pool you wish to configure site resiliency, and then click Edit Properties.
2. Click Resiliency in the left pane, and then select Associated Backup Pool in the right pane.
3. In the box below Associated Backup Pool, select the pool that you want to pair with this pool. Only the pools that are not paired with another pool will be available to select from.
4. Select Automatic fail-over and fail-back for Voice, and then click OK.
When you view the details about this pool, the associated pool now appears in the right pane under Resiliency.
5. Use Topology Builder to publish the topology.
6. Run Enable-CsTopology.
7. If the two pools were not yet deployed, deploy them now and the configuration will be completed without any issues.
However, if the pools were already deployed before you defined the paired relationship in the topology builder then you must complete the following two final steps.
8. On every Front End Server in both pools, run the following:
\Program Files\Microsoft Lync Server 2013\Deployment\Bootstrapper.exe
This configures other services required for backup pairing to work correctly.
9. From a Lync Server Management Shell command prompt, run the following to restart the lync backup services
Stop-CsWindowsService -name LyncBackup
Start-CsWindowsService -name LyncBackup
10. Force the user and conference data of both pools to be synchronized with each other, with the following cmdlets:
Synchronizing the data may take some time. You can use the following cmdlets to check the status. Make sure that the status in both directions is in steady state.
SQL Lync Back End server resiliency setup
Since the CMS is located on the sql server planning for SQL server resiliency is also very much mandatory otherwise we wouldn’t be able to get a full fledged site resiliency in a enterprise edition setup.
How ever in the standard edition this is not applicable and if the number of users are less than 3000 you can have 2 standard edition each one of them on different sites.This will allow Lync site resiliency with less roles required and much cost effective because no SQL servers are required here . Lync Front End standard edition is using SQL express installed locally.
You should use the same Back End high availability solution (either SQL Mirrioring or SQL Clustering) in both pools.i.e, You should not pair a pool using SQL mirroring with a pool using SQL clustering.
Below are the Reasons to use the same type of SQL high availability solution :
SQL clustering requires a shared storage solution, but SQL mirroring does not require shared storage solution.
SQL mirroring requires SQL witness role (in addition to principal and mirror SQL servers) for the failover of the Back End Server to be automatic. Otherwise, an administrator must manually invoke failover.
More references :
SQL clustering does not require any additional SQL servers to be able to fail over automatically-
Back End Server High Availability –
Lync 2013 high availability & disaster recovery –
Branch-Site Resiliency Requirements –
Lync Server 2010 Metropolitan Site Resiliency –
MVP – Exchange Server