Chances are that you have all you absolutely need to successfully configure and put into production the federation feature in your Lync Server 2010 infrastructure. Besides opening the prescribed firewall ports and assignment of appropriate certificates, your public DNS servers must be setup with the required A (host) as well as SRV records*. Only then can you and your peers** with on-premise Lync setup be able to automatically lookup and discover each other’s SIP domain to hit the right Lync Server 2010 Access Edge Server#.
Provided that an organization seeking to Lync federate is not on your block or black list, everything just works auto-magically thereafter. That is to say, Lync 2010 client users simply need to enter the SIP address of their correspondences in the “Find a contact or dial number” field and way they go (IM and/or audio and video, if permitted).
Problem is - what happens if your public DNS service provider or your own external DNS Servers do not support SRV records? Does that mean you are ‘doomed’ in your little island with no means to extend the wonders of Lync UC to federate with your outside contacts**?
The quick fix to this is surprisingly straightforward. To bring federation to work, manually add the Lync Server 2010 Access Edge servers’ FQDN and SIP domain on both ends of the communication points, and ascertain that the FQDNs match their respective public DNS A records. This setting can be found under External User Access / Partner domain using the Lync Server 2010 Control Panel.
For the longer term, you will have to switch your public service provider, upgrade your operating system or migrate your DNS network services and/or public URL domains to a platform that has native SRV records e.g. Windows Server 2012 Datacenter or Standard editions (both with same feature sets).
Note, though that this workaround will not function under the following conditions (that rely on SRV record availability):
- direct or open federation
- Public IM Connectivity (PIC) with Lync Server 2010 (MSN, Yahoo, AOL)
- integration with Lync On-line (Office 365, etc.)
- Lync 2010 mobile push notifications (iOS devices)
- not possible for anonymous, external users to join Lync on-line meeting
* minimum is to have sipfederationtls._tcp.<sipdomain> and optionally _sip._tls.<sipdomain>
** partners, vendors, suppliers, sister companies, etc.
# via SRV, host record, sip.<sipdomain> then sipexternal.<sipdomain> as fallback
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