The default service discovery order is:
Cisco WebEx Messenger
Cisco Unified Presence
Cisco Unified Communications Manager
Cisco TelePresence Video Communication Server
Cisco Jabber Video for TelePresence
System administrators can customize service discovery priority using DNS TXT records. Service discovery priority customization may be necessary in networked environments that provide multiple services. DNS TXT records are defined in RFC 1035. Examples of DNS TXT usage can be found in RFC 4408 (Sender Policy Framework) and RFC 5672 (DomainKeys Identified Mail).
Administrators deploying a DNS TXT record to customize service priority must use a custom form of the typical record called a Jabber Simple Configuration Priority (JSCP) record. A typical DNS TXT record has the following format:
name ttl class TXT text
A Jabber Simple Configuration Priority record changes that slightly:
name ttl class TXT JSCP-specific-text
The JSCP-specific-text parameter defines the custom service priority. This parameter contains quoted text in the following format:
"v=jscpv1 <dns-srv-name>; <dns-srv-name>; ..."
Each service is defined using the codes defined in DNS SRV record. Priority is assigned to a service by the location it appears in the service list. The first service in the list is of the highest priority and subsequent entries are of a lesser priority.
If your Cisco WebEx Messenger deployment uses Single Sign-On (SSO), the Cisco WebEx Messenger service must always be the first service in the list.
When customizing service priority using a DNS TXT record:
The priorities found in the DNS TXT record always supercede the default priority list.
The DNS SRV names in DNS TXT record are recognized by the client even if additional records are present.
A DNS SRV name with no corresponding DNS SRV record is ignored without error.
The default priority list is used and an error logged if the DNS TXT record uses an incorrect format or empty.
The default priority list is used if no DNS TXT record is found.
The following is an example of DNS TXT record with DNS SRV records and using a JSCP formatted record.
; UC DNS SRV records
_xmpp-client._tcp.example.com 86400 IN SRV 0 5 5222 xmppserver.example.com
_cisco-phone-tftp._tcp.example.com 86400 IN SRV 0 5 6970 cucm8xserver.example.com
_sip._tcp.internal.example.com 86400 IN SRV 0 5 5060 sipserver.example.com
; JSCP TXT RR example - ignore WebEx Messenger service and favor VCS service with centralized tftp over CUCM service.
cisco.com 30 IN TXT "v=jscpv1 _sip._tcp.internal.example.com; _cisco-phone-tftp._tcp.example.com; "
cisco.com 30 IN TXT "v=jscpv1 _cisco-phone-tftp._tcp.example.com"
This example is constructed so that the client ignores the Cisco WebEx Messenger service in favor of the Cisco Telepresence Video Communications Server service with centralized TFTP over the Cisco Unified Communications Manager service.
Follow these general steps to create new DNS SRV and DNS TXT records.