When deploying Cisco Unified Communications Manager (CUCM) and Cisco Unified Presence (CUPS) one of the design objectives should be to architect a solution that adds dynamic fault tolerance. One can achieve this objective by leveraging DNS SRV records.
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What is DNS SRV?
An SRV record or Service record is a category of data in the Internet Domain Name System specifying information on available services. It is defined in RFC 2782. Newer internet protocols such as SIP and XMPP often require SRV support from clients. Folks in Windows AD environments will also recognize this record as Windows uses it to advertise services.
What Does a DNS SRV Record Look Like?
First, what does the SRV record look like. Something like this:
> _sip._udp.hq-cups.netcraftsmen.net
Server: ins1.netcraftsmen.net
Address: 172.16.36.44
_sip._udp.hq-cups.netcraftsmen.net SRV service location:
priority = 0
weight = 100
port = 5060
svr hostname = hq-cups-01.netcraftsmen.net
_sip._udp.hq-cups.netcraftsmen.net SRV service location:
priority = 0
weight = 100
port = 5060
svr hostname = hq-cups-02.netcraftsmen.net
Dissecting this mess:
1. The SRV record is actually two records that point to different hosts:
- _sip: identifies the protocol (cosmetic)
- _udp: identifies the transport (actual keyword)
- hq-cups.netcraftsmen.net: the ‘cluster’ name
2. Parameters
- priority: used to specify preferential treatment for the specific host entry. Modify according to need remember, lower priority is MORE preferred
- weight: when priorities are equal, weight is used to differentiate preference. Again, keep the same to load balance. Maybe use weight and priority to provide a hierarchy of services. This could be handy in a CUPS cluster with > 2 nodes
- port: Identifies the port number to use
- svr hostname: Identifies the actual hosts A record
How Does CUCM and CUPS Use SRV Records?
On the CUCM server you create a SIP trunk that “points” to the CUPS cluster. You can use IP address (not recommended), DNS hostname, or DNS SRV cluster name.
On the CUPS server, you create a Presence Gateway and configure this gateway to point to a node in the CUCM cluster. If you are not using SRV records, it is recommended to use the FQDN of the CUCM First Node (aka Publisher node).
The CUCM and CUPS configuration guides and the Unified Communications SRND touch on the SRV configuration option but there are a few things not covered in the documentation that could create confusion. First, CUCM will default to using UDP port 5060 when the administrator configures a SIP trunk using either the IP address or DNS host name. Interestingly enough, when you check the box SRV Record CUCM will use TCP port 5060. The other point of interest is that when you specify the SRV record in CUCM you drop off the protocol and transport identifiers.
Example: _sip._tcp.hq-cups.netcraftsmen.net should be configured as hq-cups.netcraftsmen.net
With CUPS the behavior is more consistent. First, CUPS is going to use UDP port 5060 whether you use IP address, DNS host name, or DNS SRV. Second, when you configure the CUCM Presence Gateway you will use the full SRV cluster name.
Example: _sip._udp.hq-cucm.netcraftsmen.net