Ah… IOS. Refreshing after spending a week with ACME Packet’s ACLI. And at the same time it certainly has it’s own frustrations. It took me a while to get this working in my lab (partially because I was in Boston and my lab is most decidedly not), but I have a feeling DNS SRV records with SIP are going to prove very useful to me in the future.

The goal here: use DNS SRV records to minimize the number of dial-peers required in IOS. Typically CUCM is set up in some sort of redundant fashion so that there are multiple call processing servers. Going deeper, the configuration of CUCM is such that CM Groups are created with a max of three servers per group meaning everything needs to be pointed at two or three different servers in case one goes down. So we want to use one dial-peer to send a particular call to a CM Group made up of three servers rather than the usual three.

VoIP dial-peers use session targets to determine where to send a matching call. With no DNS SRV records each pattern would require three dial-peers using preference commands as shown below (Note: The use of an ipv4: target vs dns: target makes no difference here, I am using ipv4 in this example for contrast. DNS A records for each server would still require three dial-peers, it is only with DNS SRV records that we can collapse three dial-peers down into one).

dial-peer voice 1 voip
 destination-pattern 2000
 preference 1
 session protocol sipv2
 session target ipv4:10.0.0.1

dial-peer voice 2 voip
 destination-pattern 2000
 preference 2
 session protocol sipv2
 session target ipv4:10.0.0.2

dial-peer voice 3 voip
 destination-pattern 2000
 preference 3
 session protocol sipv2
 session target ipv4:10.0.0.3

With DNS SRV records we can cut this down to one dial-peer:

dial-peer voice 1 voip
 destination-pattern 2000
 session protocol sipv2
 session target dns:cmgroup1.lab.local

along with a little extra config (to host the DNS SRV records in IOS to remove reliance on the corporate DNS server)

ip host cucm1.lab.local 10.0.0.1
ip host cucm2.lab.local 10.0.0.2
ip host cucm3.lab.local 10.0.0.3

ip host _sip._udp.cmgroup1.lab.local srv 1 50 5060 cucm1.lab.local
ip host _sip._udp.cmgroup1.lab.local srv 1 50 5060 cucm2.lab.local
ip host _sip._udp.cmgroup1.lab.local srv 1 50 5060 cucm3.lab.local

ip name-server 127.0.0.1
ip domain name lab.local

Finally, in order for a CUCM subscriber server to accept a SIP URI with a destination of @cmgroup1.lab.local rather than @<servername>.lab.local each parent DNS SRV records (cmgroup1.lab.local) must be added to the Cluster FQDN list in Enterprise Parameters. If this step is not completed, expect to see SIP 404 Not Found errors. From the CUCM Help:

This parameter defines one or more Fully Qualified Domain Names (FQDN)
for this cluster. Multiple FQDNs must be separated by a space.
Wildcards can be specified within an FQDN using an asterisk (*).
Examples are cluster-1.rtp.cisco.com and *.cisco.com. Requests
containing URLs (for example, SIP calls) whose host portion matches any
of the FQDNs in this parameter will be recognized as a request destined
for this cluster and/or devices attached to it.

Many thanks to these helpful resources, the first in the list is an especially good resource:

Gateway Configuration for SIP Proxy Redundancy

Load Balance Between SIP Dial-Peers
Designing Unified CVP for High Availability
Cisco IOS SIP Configuration Guide, 15.0

Cisco SIP Gateway Troubleshooting