Lync federation also known as external connectivity which allows a lync user to connect with users in other organization as well as public IM domains and XMPP. Lync support two kinds of federation open and close. When an organization allow open federation that means the organization has enabled automatic discovery while closed means the organization has disabled automatic discovery and added specific domain to allow federation.

Lync federation can be configured from Lync Control Panel as well as from Lync Management Shell.

It is time to know “how does lync federation work?” Let’s take an example of two organizations contoso.com and fabrikam.com. Both organizations have open federation.

Below is the step by step procedure:

1. UserA (sipuri:usera@contoso.com) is looking for UserB (sipuri:userb@fabrikam.com)
2. First of all it will check the access edge configuration for federated domain.
3. As UserA request for sipuri: userb@fabrikam.com and in my case both domain is configured for open federation, the request will go to Access Edge Server and Access Edge Server will do lookup for domain name fabrikam.com.
4. If fabrikam.com domain exists then it will look for the SRV record _sipfederationtls._tcp.fabrikam.com.
5. Once it will get the destination address of fabrikam.com access edge server then it will try to establish the SIP/MTLS:5061 connection.
6. Once Fabrikam.com will receive the request from contoso.com, fabrikam.com will check the access edge configuration to verify allow/block federated domains.
7. Once the connection will be establish between both Lync Front End Pools / Lync Standard Edition Servers via Access Edge Server, UserA and UserB will talk to each other.

Note:

1. All the required ports should be open in both ends.
2. SRV record entry should exist in External Certificate.
3. Federated domain should be allowed in both ends.
4. XMPP doesn’t use SIP/MTLS:5061. It use XMPP/TCP:5269 to connect access edge server from public network and XMPP/MTLS:23456 to connect Front End Pool.