Backup registrar functionality works in Lync Server 2013 the same way that it did in Lync Server 2010. This means that the client is informed of it's backup registrar via a SIP/2.0 301 Redirect request to Home Server. This response will contain the user's primary registrar and their backup registrar:
The backup registrar information is not cached in the EndpointConfiguration.cache file on the client just like in Lync Server 2010 so the client will need this information each time in order to be able to contact the user's backup registrar. After the initial logon, the user's primary registrar will be cached in the EndpointConfiguration.cache file and all subsequent logons will read from that file. As long as the user's primary registrar is up and accepting connections, the client will register directly with the user's primary registrar, and no SIP/2.0 301 Redirect request to Home Server will be returned to the client.
With Lync Server 2013 introducing paired pools as the disaster recovery measure, it is again important that users be able to locate their backup registrar and sign in. This means that if you are going to be setting up paired pools in Lync Server 2013 make sure to use multiple SRV records that utilize different priorities:
This way clients will be able to locate their backup registrar and sign in in the event that the user needs to use their backup registrar. You can find more information on best practices in the Best Practices for Pairing Front End Pools TechNet article.