Hi,
To allow Cisco Unified Presence to discover a particular XMPP federated domain, the federated enterprise must publish the DNS SRV record _xmpp-server in its public DNS server. Similarly, Cisco Unified Presence must publish the same DNS SRV record in the DNS for its domain. As per the documentation it is mandatory to have DNS SRV records for XMPP federation
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cups/8_0/english/integration_notes/Federation/Federation/XMPPConfig_chapter.html#wp1069018
HTH
Manish