i've seen some funky behavior in my environment with users connecting to my lync 2010 server. so i went to playing around with something innocuous like forcing a GAL sync.

PS > Update-CsAddressBook
Update-CsAddressBook : Message-"The LDAP server is unavailable." LdapError-"ServerDown" "DC-01.contoso.com":"3268"

This particular DC (DC-01.contoso.com) was added a while ago to our list of DC's. We have (5) i think. This specific DC was causing issues with a new VPN technology we implemented. So we brought this DC down temporarily indefinitely.

what is the reason that this command (logged into Lync Front End) would throw this error? All related cmdlets throw the same error (Get-CsManagementStoreReplicationStatus, Get-CsAdDomain, etc). Is there a way to change what LDAP server to point to for queries?

All IP settings are correct. DNS points to correct DNS Server, DNS cache has been clea

Lync uses the available Domain Controller and if the DC is down, It should automatically switch to another. I ran in to similar issue and i had to reboot the Lync server to get this fixed.